Ryōko Ninomiya, the older sister of Shungo Ninomiya, works as a mercenary and orchestrates the relocation of Mayu Tsukimura and her brother Mikihiro to Shungo’s household, aiming to help Mayu confront her fear of men through an exposure-based approach. She engineers scenarios demanding close physical proximity between Shungo and Mayu—shared sleeping arrangements, communal baths—while documenting these interactions with a camcorder and directing participants via megaphone.

A longtime ally of Mikihiro Tsukimura, she partners with him on missions and covert plans. In the anime, she assumes a secondary identity as a school nurse to discreetly observe Mayu’s progress. Playfully manipulative, she thrives on teasing Shungo and inserting herself into absurd situations, including comedic, unwelcome advances toward Reika Hōjō.

Flashbacks reveal her history of collaborating with Mikihiro on past operations, underscoring their tactical rapport. She adapts roles as needed—overseeing Shungo and Mayu’s interactions as a film director, posing as a farmer during resource-gathering arcs—while balancing her antics with genuine concern for Shungo’s safety. She intervenes during crises, such as Reika’s volatile alternate personality episodes, blending her eccentricity with protective instincts.