Samejima, alias "Marionette King," emerges as a recurring antagonist introduced as a high school senior alongside the protagonist. A sexual deviant with predatory inclinations, he fixates on a female classmate after discovering provocative images of her, triggering an obsessive campaign to assault or manipulate her. Each calculated scheme is countered by the protagonist’s interventions, leading to his initial expulsion. Undeterred, he resurfaces with escalating desperation, deploying riskier methods that culminate in another failed plot. This defeat propels him into drug addiction, accelerating his mental unraveling.

By the narrative’s climax, his sanity fractures visibly, marked by volatile actions and disjointed behavior. His online alias, "Marionette King," mirrors his manipulative thirst for control, though later adaptations disclose his true identity. The character’s arc concludes in a violent confrontation where he suffers crippling injuries, permanently halting his predation. This defeat coincides with a pivotal shift in the protagonist’s relationship with the targeted classmate, who grapples with the trauma inflicted by his actions. His trajectory—expulsion, addiction, psychological collapse, and physical ruin—serves as an unflinching chronicle of consequence, devoid of redemption or regret.