Keisuke was raised with Akira at the Toshima Bunny Orphanage, a front for ENED's experiments. Both endured Project Nicole, training child soldiers for the Third World War, though the conflict concluded before their deployment. Later adopted into a caring family, Keisuke worked as a factory worker while maintaining his bond with Akira. His personality was characterized by low self-esteem and physical weakness, fostering an idolization of Akira's strength. Unspoken romantic feelings for Akira created internal conflict, resulting in a brief relationship he ended upon recognizing his true emotions.

Driven by concern after Akira's arrest and recruitment into Igura, Keisuke followed him to the dystopian city of Toshima. Despite no combat experience, he entered the lethal tournament to support Akira. Following repeated humiliations—nearly robbed by participant Takeru and enduring Akira's harsh rejection—Keisuke discovered the drug Line. Due to unique biological compatibility from childhood experiments, Line drastically enhanced his strength, speed, and senses, particularly smell. It also induced psychosis, amplifying his suppressed rage and obsession with Akira into violent sadism. In this state, he massacred patrons at the neutral-zone bar Meal of Duty, murdered Takeru by crushing his skull, and later attacked Akira, mutilating him with a screwdriver and sexually assaulting him.

In his character-specific route, Keisuke's fate diverges based on Akira's choices. In "bad ends," Akira's blood fails to neutralize Line's effects, leading Keisuke to disembowel him during a rain-soaked alley confrontation. When Akira's blood neutralizes Line, Keisuke recovers, regaining his docile nature but exhibiting lingering trauma like nightmares and emotional volatility. He and Akira return to civilian life, working menial jobs while navigating a tentative romantic relationship. Keisuke retains residual superhuman strength, outperforming Akira in physical labor.

Outside his route, Keisuke's arc concludes tragically. In other game paths and the anime adaptation, he dies protecting Akira—most notably bisected by Shiki's katana during Toshima's civil unrest. His compatibility with Line and its psychological toll remain consistent across all media, though the anime omits his sexual assault of Akira and alters elements like his journey to Toshima.