Mitsuko Souma is designated Girl Number 11 in the Shiroiwa Junior High School Class B that is forced into the Program. Her early childhood was initially happy, but her biological father divorced her mother and fled the country, leaving her only a keepsake ring. After her mother remarried, Mitsuko endured years of physical and sexual abuse from her stepfather, and her mother eventually joined in the abuse. At age nine, her mother accepted payment to allow three men to rape and videotape her. When she later confided in an elementary school teacher, he raped her as well. In a later attempt to be sold again, Mitsuko resisted and accidentally killed her mother. She was then sent to relatives, where the father of that household molested her. These repeated betrayals and violations left her physically, emotionally, and psychologically scarred. She developed two distinct personalities: a cold, ruthless, and manipulative persona that she presents to the world, and a fragile, childlike part that still longs for her father’s protection.

Outwardly, Mitsuko is described as one of the most beautiful girls in school, with an angelic face and a seductive smile often called a “fallen angel’s smile.” Internally, she is a hollow shell, incapable of genuine connection. She is cunning, devious, and utterly without mercy when pursuing her goals. She uses her sexuality as a primary tool, seducing male classmates before killing them, and has a warped understanding of intimacy—believing that physical contact can heal pain, a delusion instilled by her abuser. She also harbors a deep hatred of men, a result of the abuse she suffered. Her primary motivation in the Program is survival at any cost, but her actions are also driven by a need to assert control and overcome the powerlessness of her past.

In the story’s conflict, Mitsuko serves as a secondary antagonist. She claims the highest number of kills among the female students, accounting for six to eight classmates (depending on the version). She immediately begins hunting upon arrival, using her assigned weapon—a sickle or scythe—with lethal skill, and she also proves competent with a pistol. She is one of the most dangerous participants, rivaled only by Kazuo Kiriyama.

Her key relationships are largely transactional. She leads a small gang that includes Hirono Shimizu and Yoshimi Yahagi, engaging in theft and prostitution before the Program. In school, she is feared and ostracized; many girls avoid her, and she has no genuine friends. Her encounter with the wounded Yuichiro Takiguchi reveals the depth of her damaged psychology: she attempts to comfort him through sex, and when he rejects her painful advances, she kills him. Her final confrontation is with Kazuo Kiriyama, who, immune to her seduction, shoots her multiple times. In her last moments, her childlike personality surfaces as she calls out for her father, uttering that she is “Daddy’s girl.” Kiriyama kills her with a shot to the face.

Mitsuko’s character development is seen in the contrast between her ruthless exterior and the broken child underneath. Her past is gradually revealed, transforming her from a one-dimensional villain into a tragic figure shaped by relentless abuse. Though she embraces violence and manipulation, her death evokes the tragedy of a life that could have been otherwise. Her notable abilities include her physical agility, proficiency with melee weapons and firearms, and most notably her skill in psychological manipulation and feigning emotions to lower her victims’ guard.