Keiko Koshide is a central antagonist in the anime Housing Complex C. She is the wife of Seichi Koshide and the mother of Yuri Koshide, and the family relocates to the titular Housing Complex C in the seaside town of Kurosaki under the pretense of Seichi's work supervising a group of foreign fishing interns. Physically, Keiko is an adult woman with short, brown hair cut to her ears and blue eyes. On the surface, she presents herself as a polite, cooperative, and helpful neighbor, seemingly supporting her husband's efforts to manage the new residents and foster a sense of community within the complex.

This friendly and benevolent exterior, however, is a carefully constructed facade. In reality, Keiko is a sadistic and ruthless individual. She and her family are members of a cult devoted to an entity they call Kuzululu, a fallen sea god, whom they seek to resurrect through elaborate rituals of human sacrifice. Her primary motivation is to assist her husband in these murderous rites, driven by a desire for his approval and the promise of a place in the new world order they believe their god will create. She exhibits a complete lack of empathy or remorse for her victims, demonstrating a genuine, Ax-Crazy enjoyment in the violence and manipulation she perpetrates.

In the story, Keiko plays a key role in orchestrating the mysterious and violent events that plague Housing Complex C. She helps lure residents to their deaths, participating directly in the systematic abductions and murders that are made to look like disappearances. Her specific actions include befriending the elderly resident Toshi Wada to gain her trust, only to cruelly betray and kill her. She also feigns being an unwilling pawn manipulated by her husband in an attempt to lower the guard of the resident Kan, so she can try to kill him. Alongside her husband, she is directly responsible for the escalating paranoia, violence, and death within the complex.

Keiko's key relationships are defined by shared fanaticism and manipulation. Her relationship with her husband, Seichi, is not one of mutual affection but of servitude and a shared goal. When she is injured, Seichi shows callous indifference, revealing his lack of concern for her well-being beyond her utility to their mission. Her relationship with her daughter, Yuri, is similarly twisted, as she has raised Yuri to share their bloody beliefs. The family works together as a unit, but their bond is rooted in their devotion to their god and a collective glee in their gruesome tasks rather than genuine familial love. Throughout most of the series, Keiko maintains her deceptive role, but as the story reaches its climax and the cult's plans are revealed, her true nature as a remorseless killer is fully exposed. There is no positive development for her character; rather, she is unveiled as the monstrous antagonist she has always been. She does not possess any supernatural abilities; her effectiveness comes from her charisma, her skill at manipulation, and her willingness to commit extreme acts of violence. Her notable abilities are purely those of deception and murder, which she employs without hesitation.