Masumi Sekiya is a survivor of the Akatsuki incident, a ferry disaster that exposed a number of passengers to mysterious forces. She carries a cynical and guarded demeanor, shaped by the traumatic events she endured. Her primary role in the story is that of a host for the El of Water, a powerful being known as an El Lord. After being possessed, her body is used as a vessel while the El gathers enough energy to manifest independently. Once the entity leaves her, the process depletes her life force entirely, resulting in her death. Sekiya has no supernatural abilities of her own, but her connection to the Akatsuki incident makes her a link between the ordinary world and the hidden conflict involving the Lords and the Agito warriors. Her interactions with other survivors, such as Katsuhiko Sagara, highlight the lingering trauma and suspicion among those who lived through the event. She is eventually rescued from an attack by Kamen Rider Gills, but her fate is sealed by the El Lord inside her. Her brief arc underscores the cost of the Akatsuki incident and the vulnerability of ordinary humans caught in the struggle between divine beings.