Ten-year-old Yuri Koshide moves into Housing Complex C in Kurosaki with her parents, Seichi and Keiko Koshide, relocating from Tokyo. Initially appearing mild-mannered and reserved, she befriends Kimi Shirokado, adopting a protective older sister role while voicing discomfort about the complex and its residents. This persona masks her true nature and her family's hidden agenda.

The Koshide family are cultists devoted to reviving the sea deity Kuzululu. They conduct human sacrifices among the complex's residents, activities in which Yuri actively participates. She exhibits psychopathic tendencies, taking pleasure in inflicting violence, especially targeting those she mockingly calls "nicey-nices." Yuri intends to preserve Kimi's decapitated head in formaldehyde as a personal trophy. Though Yuri feigns compassion for Kimi, their friendship is a mutual deception; Yuri uses it to gather intelligence on potential victims.

During the family's ritualistic massacre of the complex's residents, Yuri chases Kimi into a hidden chamber. There, Kimi reveals her true identity as the cosmic entity Iyoyoloki Soyohosu, and Yuri sees the slaughtered remains of her parents. Desperate to survive, Yuri attempts to pledge allegiance to Kimi, offering to commit future killings in her name. Kimi rejects the plea, condemns Yuri's actions, and subjects her to a brutal death: moss violently erupts from Yuri's orifices before her body explodes.