Keiko Koshide is the wife of Seichi Koshide and the mother of Yuri Koshide in the anime Housing Complex C. She relocates with her family to the seaside Housing Complex C, ostensibly because her husband has been assigned to supervise a group of foreign fishing interns working in the area. On the surface, Keiko presents herself as polite, cooperative, and committed to supporting her husband’s stated goal of fostering harmony between the long-term residents and the newly arrived foreigners. This carefully constructed demeanor of a helpful and friendly neighbor is entirely a facade. Behind closed doors, Keiko is revealed to be a sadistic and willing participant in a horrific scheme. She and her family are cultists devoted to Kuzululu, a fallen sea god analogous to Cthulhu, and their move to the complex is part of a calculated plan to perform human sacrifices to resurrect their deity. Keiko is every bit as murderous as her husband and takes an active role in the systematic abduction and killing of the housing complex’s residents using a sacred ritual dagger.

Her motivations are rooted in her fanatical devotion to the cult and a desire for her husband’s approval, as she fears that failing to assist him would mean losing his favor. Keiko is directly responsible for the murder of the elderly building manager, Toshi Wada, whom she had befriended as part of her deceptive strategy. In another instance of her cruelty, she feigns being an unwilling pawn of her husband to let a resident named Kan drop his guard, only to then try and kill him herself. She is also implicated in placing a skinned dog’s head into the syrup for a community shaved ice party, an act designed to incite paranoia and conflict among the residents. In her role within the story, Keiko acts as a primary antagonist whose actions, combined with her husband’s, drive the escalating violence and mysterious disappearances that plague the complex. She maintains a key relationship with her daughter, Yuri, who follows the family’s cult beliefs, though her interactions with her husband Seichi appear devoid of genuine warmth, as he responds to her being injured with detached curiosity rather than concern. Keiko shows no character development away from her villainy; she remains a committed agent of chaos until her end. She meets her demise off-screen at the hands of the unassuming young girl Kimi Shirokado, who is ultimately revealed to be a far more powerful deity named Iyoyoloki Soyohoksu. The god dismembers Keiko and her husband, then dumps their liquefied remains onto their daughter as punishment for their actions. Keiko does not possess any supernatural abilities of her own, relying instead on manipulation, deceit, and a ceremonial knife to achieve her murderous goals.