M-24 was a rather large and bulky modified human who was part of the M-Series of Union experiments. He was killed by Jake, attempting to buy some time for the children. to escape. In Season 5, a revived M-24 reappears, however it is revealed he is in fact an imposter: he is really another Modified Human named Mark.
Though seen as a heartless and cruel person at the beginning of the series, M-24 was shown to be compassionate and caring towards humans, especially kids, later in the series.
M-24 and M-21 share a very close relationship as they were the only two remaining from their group. They share the same dream of finding their comrades' names. He even wished for M-21 to call him 'older brother' because he believes that previously in his life, he had younger siblings. Before his death, he told M-21 to trust 'them' (Rai and Frankenstein).
M-24 was one of the first one hundred humans that were experimented on by the Union, along with M-21. He was unwillingly experimented upon by Dr. Crombel and the Union as part of the M-Series, one of Crombel's earliest experiments. He and M-21 were classified as failed experiments within the Union. However, he had developed the unique ability of minor mind control as a side-effect of the experiment done on him.
M-21: M-21 shares a very close relationship with M-24 as they were the only two left from their group of experiments. He shares M-24's dream of learning all their other comrades' names. Additionally, M-24 asked M-21 to call him 'hyung'.
M-24 and M-21 arrive in the city with the mission to find the casket thieves have stolen from the Union. They interrogate one of the thieves and M-24 infects him after they are done questioning.
Later, M-24 and M-21 watch over for amusement as the Infected kills for survival and has to confront Shinwoo because of attacking Yuna. After the children manage to flee, the M-series duo provide the Infected with Shinwoo's address from the latter's abandoned bag, again with the purpose of entertainment.
After the Infected's failure at getting Shinwoo, M-24 and M-21 kidnap Yuna with M-24 using his mind control. They bait out Shinwoo to set up a match between him and the Infected. However, their amusement is interrupted as they sense a large amount of aura released nearby. The two of them follow its beckon and find themselves face-to-face with Rai and Frankenstein. Frankenstein warns them to stay away from the children but both M-21 and M-24 lunge at their opponents with a surprise attack. However, they are clearly outmatched. M-24's massive attack at Rai is stopped by the mere tip of the latter's finger. He is subject to the real mind control and kneels at Rai's order, unable to move a single limb. M-24's condition and Frankenstein's attacks compel M-21 to decide on retreating. As they move away from the powerful beings they have just encountered, M-24 calls off the Infected via mind link.
The Human World Arc
The Assassination Squad
M-24 accompanies his comrade whose wounds have stopped regenerating and they discuss about the identities of the people they have encountered. Their conversation is suddenly interrupted by Jake and Mary, who have come to check on them. They hurl insulting words at the duo who are considered failed experiments ready for disposal within the Union they work for. M-24 and M-21 remain silent in front of their superiors.
Afterwards Jake triggers a massacre in the hospital by the Infected and throws the blame on M-24 who is responsible for the creation of the Infected in the first place. When Jake decides to wipe out all witnesses regarding the Infected, M-24 and M-21 lies about there being none, hiding the fact of the children. However, the Infected tells them about the children and the duo have to bluff to keep them away from finding the kids.
The duo sit and watch as Jake waits for full-recovery (after his encounter with Frankenstein). M-24 has a sudden convulsion and succumbs to the regular intake of pills. Jake jeers at the pathetic fact that M-24 won't last for more than 72 hours without the pills and makes insolent comments. Despite the kindling flame of anger, M-21 and M-24 remain silent knowing their powerlessness against someone like Jake.
The two comrades spend time aloof from their overseers, deliberating on the reason why Jake's regeneration isn't working and the identity of the blonde guy who did that. The one who's worrying them now is Jake who surely wants to get rid of them and deems the organization must also want to dispose of them. They agree on keeping silent about the children. So, they contact the two unknown people they've encountered before, assuming they don't have any genuine ill-feelings towards them. A meeting follows and to their surprise, Frankenstein presents them keys to a safehouse.
Meanwhile, Dr. Crombel arrives and learning of the presence of two M-series specimens, wishes to check his old experiments. M-24 and M-21 reluctantly visit the doctor, in order to remove suspicions, after Jake and Mary disclose they have already learned from the Infected that the duo lured out the children before. They manage to temporarily evade Dr. Crombel's checking but soon tangle in the new plan of Jake and Mary.
A New Plan
M-24 and M-21 plan a getaway from the Union. They discuss the chances of trying to flee but M-24's body will break down without the pills supplied by the Union. All they can think of is to create a diversion and collect as many pills as they can from the research base and then escape. M-24 comes up with the idea of getting help from Rai and Frankenstein. He convinces his partner that although it's risky, it's much better trusting them rather than the organization. However, their plan is not put to action as Jake and Mary arrive to implement their own plan of luring out the children on them.
The children are kidnapped as both M-24 and M-21 fail to contact and warn Frankenstein of the danger. They rely on M-21's plan to use the children to lure out the 'fourth kid' (Rai).
While, M-21 and Mary wait outside, M-24 watches Jake torture the children. At one point, he is unable to bear anymore and attacks Jake to save the children. He takes them and flees. With Jake hot in pursuit, M-24 decides to stay back and hold him off, buying time for the children to flee. But he is no match for Jake and gets seriously injured. Despite that, he attempts once more to save the children from Jake who has caught up with the fleeing kids. This time, his heart is punctured by Jake and he collapses into a huge pileup.
After Rai saves the children and M-21 arrives, M-24 manages to contact with his comrade one last time via the communicating device. He advises M-21 to stick with Rai and Frankenstein and apologizes for being unable to help achieve the dream of the entire M team - finding their identities. Speaking the last words of apology, M-24 meets his demise.
Later, it was shown that M-24 was brought back to the laboratory by Dr. Crombel some time after his battle with Jake. After a big lapse of time, his body was shown in Crombel's lab again, as well as that he managed to escape from the laboratory, though was injured in the process as he had to fight many strong enhanced humans. Then it is revealed that it's actually Mark who has imbibed M-24's cells to impersonate him and fulfill Dr. Crombel's mission of infiltrating Frankenstein's Household to get Frankenstein's research data.
Even in death, M-24 was not to be rested in peace.
Like his former partner, M-24 was one of the survivors of the M-Series experiment and he was also classified as a failed experiment. However, he and M-21 began to develop and hone their new abilities in secret (for 3 years ) without informing other Union Members and unknown to them, he actually possessed a high degree of physical strength and the ability of Mind Control. It was also revealed that he had the ability to infect normal humans through his bites, turning them into monsters, and was able to maintain command over them with his mind control. Given the nature of his powers and a statement made by Dr. Crombel, it can be assumed that M-24's abilities are indeed based on those of the Nobles. Though ironically, his overall abilities (specifically his ability to infect people through a bite) are those that are traditionally associated with Vampires. Also, it has been mentioned that his body doesn't last more than 72 hours without 'pills' supplied from the Union.
As an enhanced human, M-24 has enhanced physical strength. He was easily able to destroy large walls and put up a fight against Jake who was relatively stronger than him. After his revival, it appears that his abilities have improved a great deal as he easily defeated many enhanced humans who were thought to be much stronger than him.
Mind Control: M-24's special skill is mind control. Even though it is relatively weak when compared to other Nobles, it is still effective against normal humans. It was through practicing with M-24 that M-21 was able to develop the ability to resist Mind Control.
Mind Link: M-24 was able to give orders to the Infected via his mind link. This ability is similar to the contractor-contractee mind link.