Sister Saeko runs an orphanage as a nun within a ruined city. The Koryukai crime syndicate's attempts to seize the orphanage's land draw her into conflict, compelling her to resist their encroachment. Her character embodies a striking juxtaposition: a religious vocation fused with combat proficiency, earning her references as a "ninja nun" or "commando nun" that highlight her readiness for physical confrontation. This duality extends to her fierce nickname, "Sister Psycho," signaling an uncompromising aspect of her personality. She joins protagonist Takateru Suga's team, allying with other unconventional figures including a former commando and a transvestite martial artist, to combat the syndicate. Her primary motivation remains protecting the orphanage and its inhabitants from the Koryukai's predatory actions.