Nanase Rumi transfers into Kouhei Orihara’s class, deliberately wearing her old sailor uniform despite having the new one due to personal preference. She sports long twin tails tied with ribbons—though she struggles to maintain the style—and has vivid green eyes, a slim frame, and pale complexion fitting her teenage years.

Formerly a kendo practitioner, Rumi quit after hip complications forced her club departure. This event spurred her aspiration to embody a "true maiden," adopting a meticulously polite and feminine facade around peers. Yet her authentic personality surfaces around Kouhei following their rocky first meeting: a rude, classically tsundere demeanor marked by sharp retorts and impulsive kicks when flustered. She avoids clubs but expresses literary interests.

Her past includes enduring bullying—thumbtacks on her seat, offensive chalkboard messages—with Kouhei among her rare defenders. She later forges a close bond with Mizuka Nagamori, though she periodically withdraws from classmates for solitude. In story routes, recurring expulsions from cultural clubs occur after Kouhei provokes her into angry outbursts.

Rumi’s character arc centers on persistent, imperfect strides toward femininity, frequently sabotaged by her temper. Running gags feature collisions with Kouhei during rushed school arrivals, failed club enrollments, and—in Mayu Shiina’s route—having her hair tugged by Mayu due to its resemblance to a deceased ferret. Romantic tensions with Kouhei highlight his obliviousness, like misreading her dismay over a cheap Christmas Eve date, sparking emotional clashes. Intimate moments often falter when her nerves intervene, such as abrupt TV distractions to defuse tension.

Narratives consistently explore her vulnerability when past injuries or insecurities resurface. Her interactions weave comedy—like sitting on thumbtacks she’d just warned about—with gradual emotional openness in friendships and potential romances.