M-21 is a modified human. He was unwillingly experimented upon by Dr. Crombel and the Union, leaving him with no memories of his previous life. He was originally thought to be a failed experiment until his latent powers were awakened by Rai. He currently serves as a security officer at Ye Ran High School and is a member of the group known as RK-5.
M-21 is a tall, lean, handsome (as shown when he was the center of attention of high school girls when he began working as a guard) man with silver hair that almost reaches his shoulder and almost always covers one eye. He has piercing grey eyes and a scar along the right side of his mouth. He is regularly seen dressed in a formal black suit, a white shirt, and a black tie. He wears black shoes to match and at work, the white arm band of the Ye Ran Special Security. His shirt is usually lost in battle, revealing his toned abs.
Previously, he has been seen to wear a grey trench coat and waistcoat over a white shirt, accompanied by grey trousers and shoes.
After transforming, he gains fur on the sides of his arms and shoulder blades. Also, his eyebrows and hair increase in length.
Early in the series, M-21 was shown to be a cold-hearted, shrewd and rebellious experimentee that often looked for trouble to ease his boredom. As the series progressed, M-21 started to become compassionate towards the very humans that he had wanted to kill for amusement.
He is revealed to be a very loyal man to his comrades. He was very close with M-24 and they both shared a dream of finding their comrades' names. He has shown that he really loves the children as well, risking his life to protect them. Though he doesn't show it, he is a kind man who cares for his comrades. M-21's goal is to find out who he truly is as well as the identities of his fallen fellow experimentees.
M-21 excels in provoking others to argue or fight. He practices it on a regular basis with Regis. This ability of his is put into use during his fight with Rael. He also excels in spewing diatribes and harsh words born of his directness and straight forward disposition.
M-21 is generally quiet and reserved and doesn't show much emotion. But when he loses his composure, he literally 'loses it'. His nasty temper is the sort that gives rise to serious frictions no matter what consequences await.
M-21 is one of the one hundred humans who were experimented on by the Union under Dr. Crombel. Considered to be a failed experiment, he is the last surviving early experiment in the M-Series. He slowly gained the power of minor transformation as a side-effect of the experiments done on him. Currently, he works as a security guard along with Tao and Takeo at Ye Ran High School and resides at Frankenstein's house. He is also 'Number Four' of the RK-5.
M-21 and M-24 arrive in South Korea, sent on a mission to find a casket which has been stolen from the Union. In search of the casket, they catch one of the thugs and come to the apartment where the casket was kept. However, they find it empty and M-24 accidentally infects the guy. As they can't undo what has been made, M-21 decides to have some fun watching what the infected does. This results in them meeting the children as Shinwoo manages to save Yuna from the infected. To spice up the fun, M-21 aids the infected in finding Shinwoo's address to let him have revenge. But the plan doesn't work as Frankenstein moved the children beforehand.
M-21 plans and kidnaps Yuna to bring out the other children. He uses her cell phone to call Shinwoo and bait him out for a little fun. When Shinwoo and Ikhan arrive, he calls forth the Infected and arranges a showdown between infected and Shinwoo to entertain themselves. He enjoys the fight and observes that Shinwoo is quite good when he feels a sudden aura calling them.
M-21 and M-24 abandon the battle and follow the aura to find Rai and Frankenstein who have come to rescue the children. Frankenstein tells them to stop the fight and let go of the children. But an aggressive M-21 breaks into fighting and attacks Frankenstein while his partner aims at Rai. In no time, they are outmatched and M-21 is left with scars on his hand while M-24 is confined with Rai's mind control. Frankenstein warns them to stay away from the children and walks away with his master. M-21 decides to withdraw and tells M-24 to order the infected to pull off as well.
As he and M-24 spend time together on a rooftop, they discuss about the two people they have met. The scars that Frankenstein gave him are stopping his regeneration. M-21 notices that they are really strong, so he speculates that they might be nobles. Although M-24 thinks they need to report it to higher-ups, M-21 decides not to let others know of the existence of those two. Since they are minor experiments waiting to be thrown out, they do not need to think of the Union's good. Besides, M-21 thinks they should not let union researchers know about their extended powers.
The Human World Arc
The Assassination Squad
As they talk away, two more union agents show up: Jake and Mary. They have come to check on the progress of the mission as well as taunt the minor experiments. M-21 hides all the recent happenings from them. Mary tells them to visit Dr. Crombel who wants to check them. Although they go and meet the doctor, M-21 talks out of check-ups. But soon Jake and Mary find about the infected and rounds up on them. Jake massacres a hospital and blames M-21 and M-24. He also learns from the infected about the children who are witness. Jake and Mary set on finding the children.
M-21 secretly calls Frankenstein from Yuna's cell phone which was left from the incident before. They meet up and M-21 warns him of the coming danger of the children. In return, Frankenstein provides them a key to a safehouse. This unexpected thanksgiving makes M-24 think that the two persons are truly good. He convinces M-21 in believing that they can trust those two (Rai and Frankenstein). M-21 is concerned for his pal who, unlike him, has to depend on pills from Union to survive. Otherwise, they could make a run for it and flee from the Union to live their life. M-24 suggests the next time they visit the Korean base of Union, they can grab hold of all the pills and make for it.
A New Plan
The plan of fleeing from the Union is never put on action as Jake and Mary finds the children. They also meet Frankenstein and get reasonably suspicious of him. Although the assassins intend on killing the captured children, M-21 convinces that there was 'another child' with them. They plan to use the children to bring him (Frankenstein) out. This new plan apparently seems okay but both Jake and Mary develop suspicions on M-21 and M-24. Mary attacks M-21 who uses his partial transformation to fight but is no match for his opponent. Frankenstein comes right on time to save him and stays behind to fight Mary while M-21 guides Rai to where Jake is holding the children hostage. All the while, M-24 has been holding off Jake who wanted to kill the children. And so he receives fatal wounds from Jake. Rai steps in just on time to stop Jake from killing the children. He exterminates Jake and Frankenstein bestows the same fate to Mary. M-21 is too late to save his dying pal who wants him to live on and trust the two persons. After a teary farewell, M-21 leaves M-24 disintegrating in the mass and departs with Rai and Frankenstein.
M-21 has been staying at Frankenstein's home with Rai and Frankenstein. Frankenstein questions him about the mission that had brought him and his late partner, M-24 in Korea. M-21 tells him they were sent here to search for a casket and thus came to meet them. If the mission was of higher importance than the Union would have sent better agents and not failure experiments like them. With M-24 gone, the only mission of his life is to find the identity of himself as well as his comrades which is hidden in the Union base. Frankenstein inquires about the presence of Mary and Jake, which leads to discussion about Dr. Crombel who might be present in the Union's local branch. Much to his shock, Frankenstein and Rai decide to go to the branch to meet Crombel as well as to let M-21 search for the data. Before going there, they visit the children at the hospital. M-21 is surprised to see the children and their memories replaced with thinking that they had an accident.
When they reach the base, they find it already massacred. A dying Lt. Simon tells them that it was all done on Crombel's orders who used researchers to obtain all the data on a new specimen and is now gone in his helicopter. Frankenstein leaves with Rai's permission to find Crombel. Elsewhere, Crombel sees M-21 in the security camera and tells the pilot to return to the base. At the top of the base, he meets Frankenstein. Meanwhile, M-21 and Rai enter the lab to look for data but find the improved version of the infected. The Infected attacks M-21 who seems badly outmatched and teases him for being too weak. As he is about to finish off M-21, Rai saves him but that does little to lift up M-21's spirit. Rai then tells M-21 that the origin of his power cannot be underestimated and takes some blood from M-21, calling him to awaken. M-21's heart beat changes and a full body transformation occurs. He grows claws and has a layer of fur on his body. Rai tells M-21 to show his true strength to the enemy. This turns the table and M-21 has the upperhand in the fight now. His speed, strength and ferocity increases tremendously throughout the fight and he finally kills the Infected. M-21 wonders how he got so much power in his body. He thinks that he made a contract with Rai, but Rai tells him that he didn't form any contract with him. He only showed him his true strength hiding inside and it is up to M-21 whether he can awaken it again or not.
Dr. Crombel gets to know that the infected is destroyed and so he cuts short his meeting with Frankenstein and leaves. Frankenstein returns and witnessing the destruction at the site, he finds it hard to believe that M-21 did it. M-21 goes to search for any possible information but finds that everything is destroyed. When Rai decides to leave, M-21 stand behind wondering where he will go now. Frankenstein calls him and asks how long is he going to make them wait, since Rai is waiting for him to go back together.
Back at home Frankenstein tells both M-21 and Rai what happened at the roof and that Crombel was not an easy opponent to fight against. Frankenstein asks M-21 to stay with them and adds that he can stay and leave whenever he wants, much to M-21's surprise. M-21 questions how can they trust him to which Frankenstein replies that it was Rai's decision. He offers M-21 to work at the school as a security guard. Starting from the next day, M-21 becomes a member of Frankenstein's household and steps into the school with his new job as the Ye Ran High School security.
M-21 has been carrying out his duty as a Ye-Ran security guard. Im Suyi, an idol student as well as the children's friend comes with a fellow star, Jung Hansu. Being a star, he ignores the school rules and park on the playground. M-21 takes an offensive approach as usual, telling Hansu directly to move the car. Hansu's bodyguard, Charles, comes out to teach M-21 a lesson, but M-21 brings him down with ease and even terrifies him to death. After they leave, M-21 is called to Frankenstein's office. Frankenstein tells him that he's heard what happened on the school grounds in the lunch break. M-21 is shocked when Frankenstein approves of his work and only asks him to make sure he doesn't kill anyone.
At home, M-21 is getting used to the unusual ways here. They often have the children coming over to play and party which results in Frankenstein cleaning up all mess. M-21 offers to help and ends up sticking with dish cleaning since then.
Two nobles, Regis and Seira, who have come to investigate the hospital massacre, become Ye-Ran transfer students and move into Frankenstein's house. They are suspicious of M-21 as he is a modified human present in the vicinity while the massacre itself was surely the work of a mutant. However, they don't engage in fight as they believe Rai and Frankenstein to be ordinary humans, fearing that they might harm them with their use of power.
During night, Regis and Seira go to investigate and Rai, Frankenstein, M-21 follow them from the top of a building. M-21 goes on to say that those children are Noblesse for sure and even Frankenstein and Rai are probably Noblesse too. Frankenstein tells him that he is not totally off the mark and those kids are Nobles. Then he clears up the misunderstanding regarding the word "Noblesse", which, originally is not used for an entire race but rather it's a title given to only one being (hinting to Rai).
M-21 has hard times adjusting with Regis who constantly accuses him of forsaking humanity to gain power. However, Frankenstein consoles the upset M-21 and advises him to put up with Regis and the misunderstanding for the time being.
One evening on his way home, M-21 is approached by Regis and Seira. Regis queries if he knows anything about the massacre in the hospital. M-21 tells that it was a mentally deranged person who did it. Regis reveals his doubt of him and M-21 lets out that he knows the two of them are Nobles. Regis decides to use mind control on M-21 to find out the truth but M-21 puts some resistance to finally block the power, much to the shock of Regis. Just when a fight is about to break out, Rai and Frankenstein appear and announce dinner time. M-21 and Regis continue their bickering during dinner and even afterwards. The misunderstanding doesn't clear up and M-21 wonders why Regis is holding back when he could finish a modified human like him quite easily. Frankenstein later elaborates on the ideals of nobles to M-21, clearing that as long as the two nobles think of Rai and Frankenstein as humans, they will be cautious of fighting.
The DA-5 are stationed in South Korea on the mission of finding information regarding Crombel's work there. The Union base and agents being destroyed, they find out that there is only living experiment of Crombel who can provide the information ~ M-21. Tao locates M-21 and Krantz decides to capture him alive.
One evening, M-21 is confronted by Shark and Hammer. M-21 is shocked to learn they're DA-5 members. They ask him to come along with them but he refuses. Shark tries to intimidate M-21 by releasing his aura but has to stop when Regis and Seira suddenly arrive at the scene. A fight between Regis and Shark is cut short when Krantz appears. M-21 decides his own imprisonment and leaves with the DA-5 members. Regis misunderstands M-21 again thinking he has been lying all along.
Regis has been sulking and upon returning home, he tells Frankenstein and Rai about M-21 leaving with his organization mates. Elsewhere, Krantz has been interrogating M-21 about Crombel and the base that was recently shut down. M-21 is cautious not to let out any information which leads to him getting beaten up. Krantz reveals that according to the information registered by Crombel, M-21 is on a personal assignment from Dr. Crombel himself. They leave M-21 imprisoned until he is ready to talk.
M-21 is left confused knowing what he has heard is a fabricated lie and wonders why Crombel is doing all that. He further tries to think what might Crombel have done that the DA-5 are investigating him. M-21 wonders if he should try to escape, but then decides to stay and bargain information about his comrades from DA-5 for his information on Crombel's activities in the area.
M-21 holds his ground on his bargain despite being mistreat by most of the DA-5 members. All the while Regis misunderstands M-21 as he has decided to leave with the DA-5 members.
Shark and Hammer kidnaps Regis along with the children to wipe them out as they are possible witnesses with whom Tao and Takeo has communicated. Unable to watch Shark harm the children, M-21 steps up to fight him and much to the DA-5's surprise, reveals his transformation. He buys enough time for Regis who recovers a bit too late to stop M-21 from getting the final hit. M-21 lays unconscious as Regis takes on Shark but could not finish him off due to Krantz's interference.
Rai arrives on time to save everyone and eliminate Krantz. He awakens M-21 who has been on the verge of death.
Dr. Aris Arc
After Rai's use of awakening on him, M-21 heals from the fatal injuries of his recent battle. He then requests Frankenstein to let Tao and Takeo stay with them. Frankenstein grants the request and furthermore tells M-21 that he acceded because this place is M-21's home too.
Later Frankenstein is kidnapped (willingly gone) by Yuri under Aris' order to collect a souvenir. As time pass, the household fall into worry and Rai decides to look for his servant. Rai, M-21 and Takeo leave the house in search of Frankenstein while Tao stays behind to navigate them. After a blast (from Frankenstein's battle) reveals the location, the searching party arrive there. Rai and M-21 observes Takeo dash in to stop Frankenstein from killing Teira a.k.a. Dr. Aris. After the truth is revealed and Yuri flees taking the unconscious body of the Doctor, the reunited group returns back home.
M-21 convinces Takeo to stay with them after the latter suffers the loss of his only known family (Teira), knowing it's all been a lie. M-21 calls Tao and Takeo his comrades and the trio (M-21, Tao, Takeo) form the Ye Ran Special Security.
The security trio are doing their rounds at Ye Ran grounds when they come across a noble, later revealed to be Regis' grandfather and clan leader Gejutel. M-21, Tao and Takeo receive harsh treatment for being modified humans as Gejutel considers them greedy people forsaking humanity to gain power. Regis stands up for them and clears the misunderstanding. Gejutel understands they are not people who gave up being human for acquiring power. He departs with the knowledge of Rai and Frankenstein's existence. Meanwhile, Tao initiates the formation of the RK- Raizel Knights including himself, Takeo, Regis and M-21. M-21 is assigned the no. '4' as he didn't want to join.
After Gejutel's departure, Rael arrives to take Seira back to the land of nobles. Rael who is always disgusted with modified humans, attacks M-21 in his entrance to Ye Ran. Before he could cause much damage, he is stopped by Regis and Seira. M-21 makes a quick recovery.
When Rael leaves with Seira conjuring the Lord's summon seal after having a fight with Frankenstein, Rai decides to go to Lukedonia. M-21 along with Tao, Takeo secretly gets on board the plane in which Rai, Frankenstein and Regis are traveling. But the legal travelers get off the flying plane just after reaching Lukedonia. The plane crashes into the borders of the Mergas area along with the other travelers: M-21, Tao and Takeo inside.
The trio wake up in a hospital of nobles where two guards arrive to use mind control on them. They resist the weak level of mind control and make a getaway. They help out Regis who has been busy fighting central order guards to hold them out of the forbidden area. After that they arrive in the noblesse's mansion, much to Frankenstein's surprise at how they got there. They are allowed to stay there for the time being even though the land of nobles could be assumed not to be a friendly place for modified humans.
Tao overhears from one of the Central Order guards that Gejutel is in trouble and passes the message to the household. Regis decides to make a move while Raizel and Frankenstein check out the temple. The trio follow Regis who has already started for the palace to save his grandfather from execution. Their way is blocked by Rael and M-21 stays to hold him off with a close combat so that Regis can go ahead. M-21 is aided by Takeo who covers him with long-range shootouts and Tao who guides him throughout the battle. But it is still too much for the trio when Rael uses his soul weapon and his big brother arrives the scenario. Frankenstein and Rai arrive, the latter goes ahead and Frankenstein stays to engage Rajak.
The trio follow Frankenstein and Rajak when they dash towards the palace feeling the power collision of the Lord and the Noblesse. When all misunderstanding is cleared, they are introduced to the other nobles as people whom Rai has taken under his wings.
Returning to the Noblesse's mansion, they spend a couple of days there and then head back home to resume their Ye-Ran duties.
Two new people - Sangeen and Yonsu show up in Ye Ran. They are agents of the KSA who have infiltrated Ye-Ran to test the nominees (Shinwoo and Ikhan) for KSA recruiting. M-21 starts a rue with Yonsu when they try to enter Ye Ran. Despite them joining as teachers in the school, the trio keep watch over them. They spot them connected with thugs attacking the children. M-21 gets furious and attacks Yonsu who doesn't back out from the fight either. But they are held off by their partners to avoid further collision in public.
M-21 vs Na Yonsu
M-21's power arouses Yonsu's suspicion and she steals back into Ye Ran at night to check her suspicion. There M-21 confronts a masked Yonsu and they have a fight. Both of them realize they are not normal humans and at one point Yonsu reveals her identity.
Both sides confirm that their opponent is not from the Union. So, Tao devises a deal of cooperation and the clash between M-21 and Yonsu finally appeases. KSA agents take their leave and the trio are left thinking about how to deal with the damaged school.
M-21 returns to Frankenstein's house with Takeo and Tao, who reports to Frankenstein everything that he witnessed. When Frankenstein realizes that the school was damaged, Tao shows him pictures of the damage and the trio worries comically about the principal's reaction.
Cerberus, the elite force of the 12th Elder is sent to South Korea. At one point, they attack KSA headquarters and Tao conveys the information to the household. As they investigate, An Sangeen meets with the trio. But when he returns headquarters, he is found out and attacked by Ked and Lutai. He cuts off a call from Tao to prevent bringing them into the 'inconvenience'. Tao decides that the RK team need to rescue their ally.
RK-5 vs Cerberus
The RK team arrive just on time to get Sangeen away from a fatal attack. M-21 engages in a short distance combat with the Cerberus guys while Takeo backs him up from long distance. Tao synchronizes their attack via communicators. M-21 creates a diversion for Sangeen to escape and then the head-to-head fight begins. M-21 is gradually pushed back until Regis arrives to fight alongside him. Together, they nearly bring down Ked but Rodin intrudes. The two-on-two battle proves to be more challenging as Regis and M-21 are gradually losing. Yonsu's sudden attack provides M-21 the opportunity to land an all-out attack on Ked which defeats him. Together, M-21 and Regis gradually gain the upperhand in the battle.
The 12th Elder Arc
Rodin is taking a combined attack from M-21 and Regis. Suddenly, the 12th Elder himself arrives the battle scene accompanied by Taze, the Cerberus Captain. Taze throws the unconscious bodies of Tao and Takeo which she has collected for questioning. She beats up M-21 and Regis while the Elder questions them and procures permission to kill them all. Seira, Frankenstein and Rai come just on time to save them. Duels follow, annihilating the entire Cerberus and their Elder.
After the battles end, everyone head back home where they all learn how the Noblesse is shortening his life span everytime he uses his powers to save them. Rai and Frankenstein tell them to leave as they can no longer be safe for the Union is likely to start all out war. At this, the Raizel Knights swear their allegiance to Rai, determinant to get stronger to stand by his side and defend the happy, normal life they've experienced.
M-21 accompanies Tao and Takeo to the KSA base hospital where a fugitive experiment of the Union is kept for treatment. He learns that the experiment has broken out from a Union lab and managed to escape from his pursuers. Later he is rescued from his wounded state by Rai, Regis and Seira. It appears Rai knows the experiment and he calls M-21 to have a look and the face of the pursuit too. Just as M-21 does what Rai has told him, he is shocked to see the pursuit is none other than M-24! Reunited with his long lost partner, M-21 vows to stay by his side.
Ever since, M-24 has showed up, M-21 has been spending most of his time with his former partner. Frankenstein gets suspicious but decides to take M-24 home and look after his checkup himself on M-21's request. M-24's random blackouts and health deterioration leads M-21 to ask Rai for help. But once he is reminded by Frankenstein what toll it will take on Rai, he realizes how outrageous his request is and takes it back.
Meanwhile, Tao finds out M-24 has been behaving mysteriously but keeps it secret from M-21 as per Frankenstein's order. One day, M-21 finds M-24 working on the computers of the lab. M-24 claims he just wanted to know what is wrong with himself. He even tries to make M-21 tell him about his modifications. This makes M-21 notice that M-24 is behaving differently and tells him to stop using the computers. Later Tao shows M-21 the CCTV footage clearly showing M-24 has not listened to him and has continued his doubtful conduct. This shocks and rouses M-21's suspicion too.
Gradually, with Frankenstein's treatment, M-24 gets better and M-21 takes him out on a walk. They pass the children on their way. However, M-24 does not recognize them and M-21 hides the fact that he knows them either.
Two more elders arrive at the city to investigate about the 12th Elder's annihilation. Tao and Takeo take on the job of planting false clues to frame Crombel by themselves. Realizing he needs to help out his current friends and fulfill his own duties rather than hanging around with M-24 all the time, M-21 decides to look for Tao and Takeo. Rai decides to go with M-21 and the two of them follow Tao and Takeo's trails, leaving M-24 alone in the lab (from where he escapes and is later revealed to be Mark). However, they come face-to-face with the two elders who assume them as the miscreants.
The 10th & 11th Elders
M-21 is determined to keep Rai away from fighting the elders and decides to create a diversion for Rai to flee. He causes an explosion to make a dust haze but as the smoke clears up, he finds that his diversion plan has failed ~ Rai stands rooted to his spot with a smile on his face. Rai shares with M-21 that he is the only one other than Frankenstein who has said he'd protect Rai; and it gives a good feeling. However, Rai is reluctant to take the offer and steps in to fight. Frankenstein arrives just on time with Seira and Regis to stop his master. Battles ensure the annihilation of both elders. Rai's condition worsens and Frankenstein tells M-21 and Regis to accompany his master to home. M-21 and Regis obliges and accompany Rai back home.
Tao & Takeo
Arriving at home, M-21 finds M-24 gone. Frankenstein and Seira bring back Tao and Takeo mortally wounded. But the lab is destroyed and most of the data is stolen. M-21 blames himself for everything as Rai uses his life force to awaken both Tao and Takeo despite his own deteriorating health issue. Regis condoles M-21 saying that they have no time to spare dwelling on the past. They must prepare in their own ways to gain enough strength to fight so that they do not become burdens anymore. M-21 promises to himself that he will never forgive M-24.
Traitorous Clan Leader Arc
The Raizel Knights persuade Frankenstein to train them while Rai enters a temporary hibernation. The first group of five - Tao, Takeo, M-21, Regis and Frankenstein board a private helicopter and start for the training camp spot: Frankenstein's Island. After reaching, the trainees decline the proposal of getting refreshed first and decide on starting training the first thing. M-21 offers to fight first but Frankenstein suggests a 4-1 face-off. As soon as it starts, the four trainees find themselves impaled with Frankenstein's spears.
With passing days, the trainees fail to land a hit on Frankenstein even once despite trying all formations in group attack. While patching up themselves, the four trainees discuss how they would love to hit Frankenstein even it is for once or twice. Tao receives the news of the elders' arrival from KSA and Frankenstein tells it is time to end their training and go back.
The RK team arrive home while Frankenstein has left them midway to join the battlefield that Seira is holding off alone. M-24 aka Mark communicates M-21 to bring the remaining data from Frankenstein's lab in exchange for the children they have caught as hostage. Tao makes it clear to M-21 not to brood over what he should have done and prepare to kill M-24 if necessary.
M-21 arrives at the school and comes face-to-face with M-24 aka Mark. As M-24 demands the files, M-21 receives confirmation that his teammates have ensured complete evacuation and are about to save the children. He then hands the tampered data Tao has created over to Mark. Yuri checks the data and comments it doesn't look faked. Mark contacts Kalvin and tells him to get rid of the children. Just then, the entire school lights up; the glare blocks their views and they look to find the children as well as M-21 have vanished.
Tao warns M-21 that their pursuers would catch up within 30 seconds. M-21 lets Takeo and Regis go ahead with the children to hide them while he remains to stall their pursuers. Mark, Kalvin and Yuri arrive to find M-21 blocking their way and before they could react, M-21 transforms his hand to create a blow that destroys Ye Ran building. M-21 fights with Kalvin, Mark and Yuri. Kalvin reveals that hiding children is useless as he had already poisoned them. The school building turns into a battling premise as Takeo and Regis join and take on Yuri and Kalvin while M-21 faces Mark.
Kalvin tricks the RK team into poisoning themselves. Rael arrives and helps them out under Tao's manipulation. The children are brought to the lab and kept in life support as Rai, Frankenstein, Seira and Rajak arrive after defeating the Union Elders - traitor clan leaders. Frankenstein reveals how the incident from 820 years ago had forced his master to hibernate in hiding for such a long time and only recently he has found out that his master's casket was located in the ocean all those years. M-21 realizes that the casket he and M-24 was sent to retrieve, actually contained Rai.
Tao concludes that M-24 is actually an imposter from the Assassination squad after confirming on the battle style M-21 has witnessed. Later, the trio work hard to restore the school as fast as possible and return to the normal lives they have been fighting to protect.
A state of tranquility under extreme cautions follow the precedents. Frankenstein addresses M-21 regarding the trade with Dr. Crombel. Frankenstein offers on asking Crombel for the data on M-21 and his comrades from the M-series - the names they once lost. Although excited to think about getting his dream finally realized, M-21 declines, deeming the information from Crombel must be accompanied by some demand that might put everyone's life in danger. M-21 opines that the trade should be made only regarding the children's safety. While leaving the Director's office, M-21 thinks about procuring the data in some other way by himself. However, Tao and Takeo join him to rule out his thought of handling things alone, having understood his intention perfectly. As the trio walk away together, Frankenstein, in his office with his master, concludes that it is no longer required to worry about M-21.
Afterwards, when Rai, Frankenstein and Rajak follow the trail of Rael and Regis to rescue Seira from Ignes, the trio are left to look after Ye Ran and the house. They are visited by Yuri and Mark in the school precincts, who have come to finish the unsettled matches (now that they've confirmed no nobles are around). However, they are saved by Karias who makes his entrance (a little delayed to make the perfect timing) to frighten off the enemy. Later, M-21 is left with Karias to watch the school as his teammates depart to bring back Rai and others from the island. The weirdly social nature of his colleague bemuses M-21.
Upon the household's return, the trio further fortify Ye Ran's security and are on their way to a meeting with KSA. Karias has joined them but a sudden enemy movement catches his attention and he advises the trio to leave the vicinity. They concur and leave.
However, they are caught by Kentas on their way. M-21 is awed at observing the power of a real werewolf; being in a daze, placing himself wide under attack. His teammates save him from the frontal attack and urges him to wake up from dozing off. M-21 finally defends against Kentas in his own transformed state. However, the difference in power is paramount (with Kentas enraged and determined to eliminate the 'revolting creature') and his awed state does little to help him evade Kentas' attack. He is brought down easily. Tao and Takeo go all out to provide M-21 a chance to escape as he has a dream to pursue. Unable to withstand letting his comrades die in front of him, M-21 is determined to save his friends and lets out a vast amount of aura with a howl. He finally reaches his full-transformation and faces the shocked opponent. Despite having the upperhand during the first half of the battle, Kentas soon outmatches him and decides to take him for interrogation rather than eliminating him. M-21's powers are exhausted, returning him to his normal human state. As Kentas gets ready to strike, the trio are saved by the sudden intervention of Gejutel. After the werewolf is compelled to retreat, the trio are astonished to see the Lord of nobles arrive with Karias. Later M-21 is hauled up with the help of Takeo and the trio as well as Karias are treated in Frankenstein's lab.
As soon as their serious injuries are treated, a sudden presence of Ragnarok surprises them. The trio, as well as the entire household rush to the lab where Rai has been and shockingly, finds him stabbed by the Lord. Rooted to the ground with shock, they witness the restoration of Rai's life force as Gejutel explains the Lord's intention. Afterwards, the household returns to their normal lives and M-21, along with his teammates, resumes his duties as a special security of Ye Ran High School.
M-21 is on a regular round at the Ye Ran grounds with Tao and Takeo when Tao receives an emergency call from Rael. As soon as they hear that the kitchen is destroyed, the trio recall Rai's words to be cautious with Frankenstein's belongings and perspire in fear. Upon Tao's announcement to set off to mission, M-21 transforms to a werewolf while his companions imbibe D. They accomplish the mission of fixing the kitchen, caring not to leave any trace of the wreckage for Frankenstein to find out what had happened there. Afterwards, M-21 and Takeo join Tao and witness him manipulate the two nobles - Karias and Rael to join the RK team.
A few days later, when the Lord and Gejutel have departed, the trio are confirmed to be completely healed from their former injuries as they undergo their routine check-up. In M-21's case, who has already achieved his fully awakened form, Frankenstein mentions there is nothing more he could do for him and further observes M-21 hasn't tried out that state after the fight. M-21 explains that he doesn't want to enter that uncontrollable state in the fear of hurting his friends. But Tao and Takeo assure him that they'll get stronger enough to stop him; even Frankenstein mentions he would stop M-21 himself, should it prove necessary. M-21 instantly dismisses the latter offer.
The team sets off for another training camp in Frankenstein's Island. While his comrades deal with the difficult physical and mental adjustments that come with gaining dark spear's power, M-21 spars with Karias to awaken his werewolf form. However, days of continuous sparring doesn't help him achieve the ultimate state. M-21 urges Karias not to hold back in the hope that his body might respond to danger but even winding up in a lacerated state doesn't trigger his transformation. Tao, Takeo and Regis find him barely hanging on and ardently ask to stop the training. Karias suggests M-21 to rest for the time being seeing it's no use to continue the fruitless effort. However, M-21 staggers up and decides to continue until he has achieved the awakening. Karias shows the only option that might work - a gamble of life and death. M-21 hints on his readiness. Tao and Takeo try to stop their friend but M-21 remains adamant to take on the gamble. He assures his friends that this time his chances of success are better as he still retains the senses from his recent awakening. He further tells how he heard their (Tao and Takeo's) screams when they were undergoing the excruciating pain and has decided to put his life on the line as well. Tao and Takeo finally assent to M-21's decision and convey their support. Karias brings out his soul weapon against M-21 and tells him that he had better block it with ease. As the spectators watch the arrow cut through the air and approach M-21 with blinding speed, M-21 stands still and a blue aura sphere engulfs the vicinity. When the impact clears away, everyone is relieved to see M-21 standing, completely transformed into a werewolf. Karias happily comments on his transformation while M-21 lets out a howl.
Although the transformation is successfully achieved, M-21 lacks control over it and starts attacking everyone at random. Karias steps in to stop M-21 from hurting his comrades. However, M-21 gets up immediately after taking a full hit from Karias' soul weapon. Karias orders the spectating trio to retreat to lab before M-21 becomes more angry, and dangerous. While the trio ponder, M-21 sends three massive energy waves which Karias easily manages to block. As the trio prepare to leave, M-21 lunges at them again but Karias stops him, remarking even though his power has increased massively, his attacks are rather simple due to the loss of consciousness. M-21 lunges forth and slashes multiple times but Karias easily dodges all the waves and warns M-21 that he is still too weak after the awakening because his body cannot follow suit to his instincts nor does he have any real aspiration to fight. Karias smashes M-21 into the ground and the fight draws to an end.
Later, M-21 undergoes a checkup session with Frankenstein who comments on the improvement (also reproaching Karias for the harsh training). There an information from Seira temporarily stupefies them: a possibility of Rai being sad over Raskreia's absence. A sudden call alerts Tao about the recent disturbances in the city, and the trio immediately embark.
The trio arrive at the battle scenario just on time to announce themselves as change of fighters for battling Gaitan. They find Rael and Regis in a worn-out state, having battled Grui until that time and decide to take on the werewolves by themselves to give the young nobles a bit of rest. With the commencement of the battle, M-21 transforms to his werewolf self (retaining his consciousness and attire) while his partners (Tao and Takeo) have also summoned the dark power imbued in them. M-21 starts off with a frontal charge followed by a series of punches which are easily blocked by Gaitan. He is sent off flying but is backed by a shower of special ammos from Takeo which hits Gaitan directly. As Tao holds Gaitan captive in mid-air, M-21 again charges at the werewolf and Takeo accompanies with his shots. The triad's attack, however, fails to do any apparent damage as Gaitan emerges unscathed but excited to find such interesting people in the vicinity.
M-21 resumes his duties as Ye Ran special security and continues protecting the students. One day, Aris pays a visit to the locality and sets her eyes on Shinwoo as a fine specimen. Yuri, who accompanies her, is about to approach to pick up the specimen to pacify her when M-21 enters the scene.
M-21 reminds Yuri of the truce between Crombel and Frankenstein. However, both Yuri and Aris don't consider the "failed and disposable" experiment as a threat. M-21 begins to provoke them. Yuri is confident that they are capable of easily defeating M-21 if he is alone. M-21, who is mildly amused, further provokes them by daring them to kill him. They change the location for battle and Dr. Aris decides to teach "trash" agent a lesson. She launches attack on M-21 but all of them are shocked to see M-21 in his more 'evolved' Werewolf form. M-21 brings down Aris in one single punch and also defeats Yuri within a few moments. Mark, left alone, is visibly scared by M-21. Before he could think further, M-21 is behind him and asks him why he is still pretending to be M-24. M-21 remembers his last encounter with Mark and is sure that he is not his fallen comrade. M-21 trashes Mark for offending the memory of his treasured friend. He then takes a moment to reminisce his past life as experiment specimen - the death of fellow M-series members, M-21 and M-24 failing in enhancements, being bullied and labelled as "trash".
Aris gains consciousness and expresses her jealousy that a failed experiment like M-21 could get such powers. M-21 gets furious and is about to kill her who hardly knows the pain and suffering he underwent for that power he did not even want. Before he could kill her though, Muzaka interferes. He remembers M-21 as a kid under Rai's protection. He is impressed by his powers which resemble that of his own race. Tao and Takeo enter the scene as well. Muzaka senses their improvement and acknowledges that it is because they have been under Frankenstein's care. After a short conversation of both parties (Muzaka & Tao), the trio let the matter go while Muzaka takes Aris, Yuri and Mark and leaves.
A while later, M-21 finds another visitor wandering around in the vicinity. It is Kentas. M-21 crosses him but Kentas makes it clear that he does not intend to create a scene (which will alert his pursuers). M-21 explains that he is just doing his guard duty and as long as Kentas does not stir up any trouble, he can do what he wants. Kentas looks around for his ex-Lord, Muzaka, allowing M-21 to follow him after he destroys his communication device. They wander around for days. Finally, Kentas stops and asks M-21 whether any werewolf has been in the area recently. M-21 tells him about Muzaka's frequent visits and in turn learns from Kentas that his own aura in transformed state is identical to that of a real werewolf. The aimless journey continues. Kentas settles on a rooftop, at a loss how to meet Muzaka now that he knows he comes there. M-21 returns with some burgers and passes one to Kentas. They soon start talking without hostilities over their meal until they are interrupted by the sudden arrival of Juraki and Gayare.
Juraki reveals that they have come to take Kentas back to their Lord and tells Gayare to dispose of M-21. Kentas tries to prevent involving M-21 but the battle begins anyway. M-21 realizes that his opponent is very strong. So, he holds back to make Gayare underestimate him. When Gayare lets his guard down, M-21 reveals his transformed form and delivers a few powerful blows to make his getaway. But twin werewolves, Urne and Mirai stop him midway until Gayare catches up, swearing to kill him. M-21 is easily overpowered. Gayare launches his finishing blow but much to the shock of everyone, M-21 survives. Kentas requests again to spare M-21. This time, Juraki consents. But he intends to take the modified human captive to their lair to find out how he could have powers identical to that of a warrior werewolf.
M-21 and Kentas are taken to werewolf territory and confined in a lab for experimenting. While under supervision of Juraki and Gayare, Kentas learns from them that weak members of their race as well as transgressors are used for experiments there. Kentas could hardly believe how low his race has stooped for power. Juraki and Gayare defend their reason for experimenting. M-21 wakes up from his comatose and gives them a piece of his mind (remembering his own harrowing experience as an experiment specimen for the Union) by comparing them with humans who use others of their kind as they please to satisfy their own greed while using "necessary sacrifice for the future of their race" as an excuse. M-21 chides the werewolves by concluding "No matter how well one attempts to sugarcoat it, a madness is a madness."
Ignes Kravei, who is in charge of the experiments, appears and asks Juraki and Gayare to leave. Realizing that they are in the mercy of a madwoman, M-21 braces himself for the oncoming pain and asks Kentas to do the same. All kinds of brutal, torturous experiments follow. Ignes is delighted by the results her new experiment specimens have yielded, as well as the fact that they have survived unlike the past specimens that she had to discard. M-21 suffers a comatose state for three days and remembers his worried comrades back home after regaining consciousness. Unknown to him, Rai and Frankenstein are already on their way to rescue him.
M-21 and Kentas barely hold on to dear lives as Ignes' experiments keep getting brutal. Kentas wonders how the modified human is still alive since his own body has reached its limit. M-21, although alive, is in no better condition than Kentas. Kentas expresses his disappointment towards the warriors who are using weaker members for experiment. M-21 then opens up to him about his own life as an experiment. A while later, two werewolf scientists come to the lab who share the same views as Kentas. They free Kentas and M-21 despite knowing what punishment awaits them, should they be found out. M-21 and Kentas, now free co-specimens, finally introduce themselves to each other.
To avoid getting caught, M-21 and Kentas hide in the labs as no one would suspect them of hiding near the place they have just escaped from. They stumble into a room full of dead werewolf specimens confined in tanks. Kentas is distraught after witnessing the misery of weak family members who he was supposed to protect as a warrior. M-21 is shocked too and wonders what types of grotesque experiments were being carried out. He advises Kentas to help the werewolves he can instead of throwing away his life by confronting his clan. Kentas finally regains his senses and thanks M-21 for his sensible advice. Meanwhile, Ignes learns of the escape of her two specimens but is confident that they'll be caught soon enough since they are physically exhausted.
M-21 and Kentas destroy the labs before escaping. They express camaraderie which has grown during this time of crisis. But now, they must part ways. M-21 recalls the worried household and is about to leave, however, before he can go further, Maduke's warriors catch up. Kentas desperately attempts to buy M-21 some time and engages in a battle. Sadly, M-21 does not go very far either: for he runs into Ignes. After a futile attempt to fight back, M-21 once again falls prey to the vicious researcher.
Ignes drags a mutilated but conscious M-21 towards the lab and stops on the way where Kentas has been fighting Gayare. M-21 reminds Kentas, who is on the verge of losing, of his duties as a warrior. He is quickly brutalized into silence by Ignes. But his words get Kentas back on his feet. Ignes notices that the werewolves are irritated at her presence and decides to leave, taking M-21 with her back to her lab, intending to take all her stresses out on him.
M-21 is once again held captive in the lab and tortured with experiments. He is found by Lunark, who has come in search of Kentas. She notices M-21 as the guy Frankenstein is looking for...Initially hesitant, she frees M-21. But she fails to rouse him because of his severe injuries. Before she could take M-21 and getaway, Ignes confronts her to prevent 'her property' from getting stolen, however, the battle is cut short at the mention of Rai's presence and Ignes withdraws.
Later M-21 regains consciousness while fleeing with Lunark and is shocked to learn that Rai and Frankenstein has come to rescue him. He is worried as he recalls Frankenstein's explanation that Rai exhausts his life force every time he uses his power. This had led him to realize the price of his awakening, and he blames himself for compelling Rai to use his power again. Lunark asks M-21 about his relationship with the Noblesse, seeing that the Noblesse didn't hesitate for a moment to come to the werewolves' lair. M-21 has a flashback where Rai asked Frankenstein if they were a family. M-21 answers Lunark that they simply live together. But when Lunark demands a serious answer, M-21 replies that to Rai, they are a FAMILY.
Regis K. Landegre: M-21 and Regis start off in bad terms with each other. M-21 used to detest the prideful, young noble. However, it changes after seeing each other risk their lives for the children. Both of them eventually gain each others trust and share camaraderie.
M-24: M-21 was very close with M-24 as they were the only surviving members of the M-series. He also joined Frankenstein's household because it was M-24's last request.
Frankenstein: M-21 had some initial clashes with Frankenstein; however, gradually both men seem to respect each other. Frankenstein also allows M-21 to call his residence 'home'. M-21 is usually cautious around Frankenstein but becomes somewhat more fearful of Frankenstein after seeing his fight with Rajak.
Tao: M-21 shares camaraderie with Tao. Albeit, he considers the latter quite eccentric but at the same time, he knows he can fully depend on his direction (especially during battles).
Takeo: M-21 also considers Takeo as his comrade. They are the 'dish-washing duo' since that has been their fixed post in the kitchen ever since joining the household.
Earlier in the story, M-21 was shown to have little power compared to the other modified human agents of the Union, despite manifestations of abilities beyond what was expected of him. This changed after his latent powers were awakened by Rai and he is shown to possess considerable potential, which he temporarily manifests in a very powerful transformation. This is later revealed to be due to his heart actually being that of a werewolf. Though temporary, that single transformation increases his basic abilities to levels far beyond what they used to be. Under Frankenstein's care, he works on stabilizing and improving his physical abilities and gradually becomes capable of voluntarily accessing his transformation at any given time. At first, he is able to do partial transformations for limited period of time. Gradually, his overall abilities become powerful enough for him to pull off full transformation. He even single-handedly defeats Aris, Yuri and Mark with relative ease. His intense training with Karias and Frankenstein results in him learning to perfect his transformation, so much so that even werewolves admit that his aura is identical to their clan members'. Kentas acknowledges it after seeing M-21 transform while fighting Gayare. He also thinks that M-21's strength is not to be underestimated, considering the combat experience he had fighting opponents stronger than himself. The werewolf twins, Urne and Mirai thought that he was a Union Elder after witnessing his power and commented that their clan could use someone as strong as him. He has become so powerful that Ignes Kravei has compared him to werewolf warriors.
M-21's heart is actually that of a werewolf, which was implanted into him during the M-Series experiments. This is the source of his transformation abilities. Rai has mentioned that the origin of his power should not be taken lightly.
Having the werewolf heart and modifications on him, M-21 can transform into a werewolf. However, he could not transform completely at first. He started with the ability to change his nails to claws when he worked in the Union. This ability gradually improved after being awakened by Rai and he was able to transform his lower arms. Gradually, he could transform his entire arms along with a part of his chest area. When his comrades Tao and Takeo face mortal peril to save him from Kentas' attack, he partially attains his full transformation releasing aura and a howl. However, he tends to lose consciousness after transformation attacking friends and foes alike. After training with Karias, M-21 has learned to retain his consciousness after a proper transformation. It is now to the point where even true werewolves do not see the difference between themselves, stating that M-21 is nearly as powerful as them.
M-21 possesses incredible physical strength. He was able to block Na Yonsu's enhanced punch easily with one hand. Later in his battle with Cerberus, he destroyed a building with a single powerful attack. His strength grows after, to the point where even werewolf like Grui recognize it as on par with them. He is also very fast, able to almost match Rael in speed with assistance from Tao and Takeo. More recently, his speed enabled him to temporarily keep up with a Cerberus agent without Takeo and Tao's assistance.
Regeneration: M-21 has displayed incredible levels of regeneration on several occasions. He is shown to heal quickly after being injured by a noble, which is unusual due to the fact that their attacks are imbued with their power, thus blocking or slowing down regeneration. He is able to survive injuries that would normally be fatal, even to a modified human.
Mind Control Resistance: According to M-21, he used to practice to resist Mind Control with his deceased comrade, M-24. This is further enhanced after he's partially awakened, allowing him to resist Regis' Mind Control and on a later but less impressive occasion, against the Mind Control of a Central Order investigator.