Miya Satsuki presents herself as a cheerful and friendly high school student, particularly towards classmate Juu Juuzawa. She persistently seeks his company during lunch breaks, maintaining a positive demeanor despite his indifference. This outward behavior masks a traumatic background involving severe psychological manipulation.

Her descent into criminality began when neighbor Kiyoshi Kakura, a serial killer claiming to be a "covert government operative" eliminating disguised aliens, raped her. Developing Stockholm Syndrome, she became emotionally attached to Kakura, viewing him as an older brother. She joined his murders, serving as an accomplice by photographing victims immediately before and after Kakura attacked them with a baseball bat.

Her role evolved from documentation to active participation. Suspecting one victim remained alive after Kakura's assault, she used a piece of wire to strangle them. This act awakened her sadistic tendencies, leading her to routinely finish off victims by strangulation. She thrilled at capturing their final moments and anonymously posted these photographs on online message boards.

Her criminal activities culminated in an attempt to murder Juu Juuzawa. Classmate Ochibana Ame intervened and subdued Satsuki with a taser. After briefly recovering, Satsuki attacked Ame with a boxcutter, but Juu took the injury. Ame then tased Satsuki again, rendering her unconscious. Authorities arrested Satsuki; her cooperation led to Kakura's arrest. Kakura later committed suicide in prison.

Physically, she has orange or brown shoulder-length hair, brown eyes, and a typical school uniform appearance. Psychological assessments describe her as exhibiting traits including loyalty to Kakura, skepticism towards others, and anxiety about safety, noted as informal observations rather than clinical diagnoses.