Rin Misumi enforces school rules as head of Seifu High's disciplinary committee, initially clashing with Ui Wakana's "liberation of love" campaign. Her strictness originates in personal history; she joined the committee to refute rumors accusing her of using her large breasts to seduce boys, seeking to demonstrate her moral integrity instead. This duty fuels ongoing tension with the student council, especially Ui.

She resides with her older sister, Kei Misumi, the school nurse. They later relocate to an apartment adjacent to student council vice president Hayato Izumi. Rin develops romantic feelings for Hayato following his consistent kindness and assistance during various incidents. These feelings escalate into intense jealousy after Kei accidentally discloses Hayato and Ui's engagement and cohabitation.

Throughout the series, Rin undergoes significant development. Initially rigid and disapproving of public affection, she gradually becomes more accepting of Hayato's attention to her appearance. At a summer festival, she confesses wearing a minimizing bra due to self-consciousness about her breast size. Later, she allows Hayato to touch her breasts, marking increased comfort with intimacy. Her exceptional culinary skills frequently facilitate interactions with Hayato, deepening their evolving dynamic.

A subplot reveals Rin was a childhood friend to both Ui and Hayato, though this connection receives no further exploration. Her popularity surges, particularly among incoming freshmen drawn to her kindness and style, when she joins the main cast's class in the second year. Rin's narrative prominence often overshadows Ui in later manga arcs, centering on her romantic rivalry and disciplinary role. Series marketing materials frequently highlight her imagery, reflecting her sustained appeal.