Umi Natori is Kaizō Katsu's childhood friend, irrevocably altering his life at age seven when she kicked him off a jungle gym. This incident caused severe head trauma, warping his perception of reality and sparking his obsession with the occult. Umi carries deep guilt over this event yet consistently avoids admitting responsibility.

Initially appearing relatively normal among her eccentric science club peers, Umi gradually reveals disturbing undercurrents: violent tendencies and a habit of cursing others. She harbors romantic feelings for Kaizō, frequently expressing them through tsundere-like aggression. Her childhood established her as an Enfant Terrible, taking children's games to literal and extreme ends—like performing actual surgery during pretend scenarios.

Umi chronically misreads social contexts and situational moods, causing discomfort. Training aimed at correcting this social blindness yielded little improvement. She becomes entangled in increasingly absurd events, including a fabricated pregnancy plot where Suzu Saien manipulated Kaizō into believing he impregnated Umi while intoxicated.

The conclusion reframes Umi's existence, revealing she and Kaizō were institutionalized psychiatric patients. Their shared delusion, guided by Suzu—their actual therapist—constructed the series' events. Their recovery culminated in leaving the mental facility and establishing independent cohabitation.