Keiko Koshide is the wife of Seichi Koshide and mother of Yuri Koshide. She relocates with her family to a seaside housing complex as her husband supervises foreign fishing interns. Outwardly polite and cooperative, Keiko maintains a facade supporting the family's stated mission of fostering harmony between residents and newcomers. This carefully constructed demeanor hides their true allegiance.

Keiko and her family secretly belong to a cult devoted to Kuzululu, a fallen sea god analogous to Cthulhu. Their move is part of a calculated plan involving human sacrifice to resurrect this deity. Keiko actively participates in the systematic murder of housing complex residents for this ritual. She is directly responsible for killing the building manager, Toshi Wada. Her actions, combined with her husband's, drive the escalating violence and deaths within the complex.

Keiko and Seichi meet their end off-screen at the hands of Kimi Shirokado, later revealed as the powerful deity Iyoyoloki Soyohoksu (Yog-Sothoth). Kimi dismembers them. Their liquefied remains and organs are then dumped onto their daughter, Yuri. This fate underscores their role as antagonists and the severity of their crimes against the residents.