Sister, an enigmatic child found with her Brother inside a Shinkansen vault during an Akudama-led heist, is initially presented as a "gift" bound for Kantō. The siblings’ true nature emerges as immortal entities with advanced technological enhancements traced to the Kyushu Plant, a facility harnessing infinite resources.

Her striking appearance features black hime-cut hair with braided accents, crimson eyes, and a traditional ensemble blending a white kimono, deep-red hakama, and rugged brown boots. She wields a flute that projects protective force fields against environmental hazards or combat, alongside a voice-activated lunchbox capable of molecularly synthesizing meals from ambient elements.

Inherently reserved, Sister clings to her Brother, relying wholly on his guardianship. Their symbiotic dynamic fractures during the Akudama mission, triggering her terror of abandonment until the Swindler intervenes, becoming her reluctant protector. Gradually, Sister mirrors the Swindler’s mannerisms, forging a fragile trust that transitions her dependency to a surrogate sibling figure.

Her unexplained regenerative powers enable recovery from catastrophic injuries, including decapitation, cementing her immortal status. The siblings’ final ordeal forces Sister into a rocket with the Swindler, who sacrifices herself to safeguard her. Stranded yet resilient, Sister adapts, embracing the Swindler’s guidance until her demise. Post-cataclysm, Sister and her Brother flee Kansai, traversing a tunnel to Shikoku—a tentative emblem of survival.

Rooted in themes of vulnerability and systemic oppression, Sister’s journey reflects the corrosion of innocence within a fractured society. Her cautious shift from passivity to nascent autonomy underscores the narrative’s interrogation of morality in a dystopian framework, anchored by her bonds with both Brother and Swindler.