Sayuri Hida serves as the top-ranked combatant in the student council's Assault Squad, wielding a bamboo practice sword with deadly efficiency. She masters the Hida Kasseiryuu swordsmanship style, a testament to her heritage as a recognized genius from the pure-blooded Hida lineage. Severe visual impairment defines her physicality; without glasses, she becomes nearly blind and dysfunctional, often frantically groping while repeating "my glasses, my glasses"—a vulnerability frequently exploited in her stories.

Quiet, shy, and introverted, Sayuri maintains a stoic, monotonous demeanor. Yet beneath this reserve lies fierce loyalty to student council president Kanade Jinguuji and fellow Assault Squad members, especially captain Nanaho Kinjo. This loyalty coexists with independent conviction, leading her to openly challenge Nanaho over moral disagreements. Her closest bond is with exuberant childhood friend Rein Tsunomoto, whom the Hida family adopted after Rein's father vanished. Rein's extroversion contrasts with Sayuri's quietness, and she often voices envy toward Sayuri's ample bust.

A past swordsmanship accident haunts Sayuri: she inflicted permanent injury on a senior, burdening her with profound guilt. She atones by sending daily flowers to the injured senior, a secret initially shared only with Rein. Over time, with Rein and friends' support, she confronts this trauma, progressing from emotional isolation toward healing and principled assertiveness within her team.