Kasumi Ishiki, a Japanese teenager infected with the lethal Medusa virus, grapples with survival alongside her twin, Shizuku. Selected for cryogenic preservation while Shizuku remains untreated, Kasumi battles crushing guilt, believing her sister more worthy of salvation. Shy and self-doubting, she contrasts Shizuku’s confidence, clinging to her with near-obsessive dependency—a bond strained when Kasumi’s suicide attempt, thwarted by Shizuku, exposes her fragile psyche.

Decades later, Kasumi awakens in a derelict facility consumed by mutated flora and predatory horrors. Forced to shed her timidity, she emerges as a resilient leader, guiding survivors through peril. Her bond with Marco Owen, a brusque yet capable ally, deepens into trusted affection despite skepticism from others about his motives.

A cliffside confrontation unveils a devastating truth: the original Kasumi died years prior. During a failed suicide pact with Shizuku, her fatal fall activated Shizuku’s latent Medusa abilities, which grief-strickenly manifested a duplicate Kasumi. This clone, unaware of her artificial origin until the narrative’s climax, inherits the original’s memories and emotional scars.

Struggling to reconcile her fabricated existence, Kasumi confronts her codependence on Shizuku. Her growth peaks when she chooses autonomy after Shizuku releases control. The manga concludes with Kasumi and Marco escaping the collapsing facility, their future ambiguous yet hopeful. The film adaptation diverges, pairing her with Tim, a young survivor, as she pledges to fulfill Marco’s dying wish by cherishing life.

Visually, Kasumi sports short black hair, round glasses, and media-shifting eye colors. Her initial pink cryogenic uniform, later torn and replaced by a black dress and white shirt, mirrors her transformation from fragility to resolve.

Through harrowing trials, Kasumi’s arc traces survival, fractured identity, and the scars of trauma, culminating in her hard-won embrace of self-reliance amidst chaos.