Nanako Matsutaka is a university student initially depicted as psychologically typical. She joins the Abnormal Physiology Seminar motivated by romantic attraction to fellow student Komugi Musashi, fundamentally seeking to maintain her mental stability amidst peers who openly discuss taboo subjects like sexuality and fetishes while exhibiting unconventional behaviors.

The seminar environment catalyzes her development through sustained exposure to classmates devoid of shame regarding their perversions, including Komugi’s netorare fetish and Miwako Mizukoshi’s masochistic tendencies. Originally positioned as the group’s sole rational member, her resistance gradually erodes. Documented behavioral shifts include developing an appreciation for atypical sensory experiences—such as the smell of her own flatulence in the bathtub—and exhibiting exhibitionist tendencies, like beginning to undress during a blackout at a party. These changes signify her incremental normalization of aberrant topics and deterioration of initial moral boundaries.

Her relationships critically influence this transformation. Her attraction to Komugi persists despite his manipulative behaviors—such as fabricating academic justifications for stealing panties—and open fetish discussions. Interactions with Miwako, Komugi’s ex-girlfriend, introduce psychological complexity through their contrasting sexual attitudes. Professor Kenji Meshiya’s assignments—focusing on topics like indirect kissing, panty theft, and the hedonistic aspects of taste—systematically challenge her conventional mindset.

Nanako’s trajectory progresses from confusion and resistance toward reluctant participation and eventual internalization of the seminar’s values. This culminates in her compromised status as the "only sane man," marked by an uneasy coexistence of residual normalcy with newly acquired tolerances and behaviors.