Momomi Momoi is the daughter of Momokichi Momoi, a mob boss who owns the Peach Twisters baseball team. Raised within her father's criminal world, she normalized violence as a tool for handling failure, exemplified by his execution of underperforming players. Momomi exhibits extreme and impulsive violence, killing others without hesitation. Rumors claim she committed her first murder at age ten to celebrate her birthday and favors the color "blood red," decorating her room with "psychotic panda" motifs.

She is in a lesbian relationship with Sakura Sakarada. When the Peach Twisters' manager and pitching coach threatened to expose this relationship, Momomi killed them to prevent scandal. Following her father's kidnapping by the Danger Service Agency (DSA), she executed his bodyguards for failing to protect him. She then spearheaded a city-wide manhunt for the DSA, culminating in Mikura Suzuki's capture.

Sakura Sakarada later reveals Momomi is Mikura's half-sister; both share a precognitive ability, experiencing involuntary visions of the future. Momomi experiences two visions involving Mikura, hinting at an unresolved, interconnected fate. During a confrontation after a staged hostage exchange, Momomi is ambushed and killed by Mikura, concluding her narrative.