A stern, rule-following, bespectacled instructor at Futatsubashi Elementary School, Sae Shirai has garnered a fearsome reputation among students and staff. With both parents being teachers—one now the chief of the board of education—it might seem natural for her to follow in their footsteps, were it not for her deep-seated dislike of children and aversion to social interactions.
In her youth, quiet and studious, Shirai endured relentless criticism from her mother and was dismissed by peers as gloomy, serious, and plain. This led her to become bitter, antisocial, and resentful towards the world. Consequently, she adopted a highly disciplined, traditional teaching style that contrasts sharply with Daisuke Aoki's enthusiastic, positive approach, often causing conflicts on numerous topics. Her personal life is virtually nonexistent; despite nearing thirty, she has neither dated nor had sexual relations, which remains a significant source of frustration for her.
After losing her temper at Kuro Kagami for mocking Aoki about his virginity (unintentionally revealing her own status), Shirai was surprised when Kuro later approached her, offering fashion tips, appearance suggestions, and even friendship under the nickname "Shiro-chan." Though initially annoyed by this sudden attention, she surprisingly tolerated it. When Kuro sought advice on handling her unrequited love for Rin, Shirai inadvertently hurt Kuro’s feelings by implying that her lesbian attraction was abnormal. Only after a series of personal events did Shirai realize the depth of her mistake and her poor judgment.
To make amends, Shirai wore a white hairpin gifted by Kuro. She felt flattered when Kuro defended her by physically confronting a male student who insulted her appearance, marking a turning point. Although not transformed into a social butterfly, Shirai showed signs of softening, such as breaking her own rules to comfort a heartbroken Kuro during an athletic meet. Recently, she began reevaluating her life, particularly how much she resembles her despised mother, and repaid Hōin’s favor by helping her deal with a belligerent parent-teacher association leader, finding some inner peace. There are subtle hints that she may be developing feelings for Aoki after an awkward train encounter, though she adamantly denies it.
