M-21

M-21 is a tall, lean, handsome (as evidenced by his popularity among the female students when he began working for the school's security) man with silver-gray hair that almost reaches his shoulders and almost always covers one eye. He has piercing gray eyes and a jagged scar on the right corner of his mouth. He is often seen wearing a formal black suit, a white shirt, a black tie, and black shoes. When on duty, he wears a white armband, a sign of Yeran Special Security. His shirt is usually lost in battle. At the beginning of the series, when he was tracking Han Shinwoo with M-24, he wore a gray trench coat and vest over a white shirt, along with gray pants and shoes. After transforming, he gains fur on the sides of his arms and on his shoulder blades. His eyebrows and hair also become longer. He resembles Muzaki's transformed state when he transforms.

At the beginning of the series, M-21 was shown to be a cold-hearted, shrewish, and rebellious experimenter who often sought out trouble to relieve his boredom. As the series progressed, M-21 began to feel compassion for the very people he wanted to kill for fun. He was revealed to be a very loyal person to his comrades. He was very close to M-24, with both of them dreaming of finding the names of their comrades. He showed that he also truly loved children, risking his life to protect them. Although he does not show it, he is a kind person who cares for his comrades. M-21's goal is to find out who he really is, as well as the identities of his fallen experiment mates. M-21 is adept at provoking others into arguments or fights. He regularly practices this with Regis. This ability of his was put to good use during his fight with Rael. He also excels at spewing diatribes and cutting remarks, born of his forthrightness and straightforwardness. M-21 is usually quiet and reserved and does not show much emotion. But when he loses his temper, he literally "loses it." His nasty temper is the kind that leads to serious friction, no matter what the consequences.

M-21 is one of the hundred people experimented on by the Union under the direction of Dr. Crombel. It is believed to have been a failed experiment, but he is the last survivor of the early M-series experiments. As a result of the experiments performed on him, he gradually gained the ability to undergo minor transformations. He currently works as a security guard alongside Tao and Takeo at Ye Ran High School and resides in the Frankenstein House. He is also the "number four" of RK-5.