M-21 is one of the first one hundred humans that was experimented on by the Union. He is designated as the 21st human in that batch. He was classified as a failed experiment, and took the lowest raking agent position in the Union along with M-24.
He is the last surviving early experiment in the series. Early in the series, M-21 was shown to be a cold-hearted, rebellious experiment that often looked for trouble to ease his boredom. He began developing the ability to transform his physical shape which he kept a secret from the Union, as they would undoubtedly continue their experiments on him. His closest friend and fellow lab rat, M-24, created the goal with him to try and discover the identities of themselves and the other test subjects, as they have no memories before the experiments. After M-24 died defending Shinwoo and his friends from Jake, M-21 moved in with Frankenstein and Rai. Rai assists M-21 by briefly allowing him to glimpse his own true powers, telling M-21 that he must not underestimate the origin of his powers.
He then took up the position as a Special Security Guard in the school that Frankenstein runs. Eventually, he makes peace with the death of M-24, letting go of his goal to discover their true identities in favor of protecting Shinwoo & co. from the Union. Thanks to Tao, M-21 discovers that his defection from the Union was never reported by Crombel, and all the data regarding the test subject's identities had been destroyed recently after being untouched for so long. This data purge happens, most probably, when Crombel discovers that M-21 and M-24 may not have been complete failures as experiments.
During a health checkup, Frankenstein reveals that M-21 had been quite different from his fellow "M" series subjects: M-21 had extensive gene modification and had been implanted with the heart of a werewolf, a species nearly as powerful but far more obscure than the Noble's.
