A strict, by-the-book, bespectacled instructor at Futatsubashi Elementary School, who has gained a reputation as a fearsome authoritarian figure among students and staff alike. As the daughter of two teachers — one of whom has since gone on to become the chief of the board of education — it might come as little surprise that Sae Shirai would become one herself were it not for the fact that she hates children and eschews social interaction. Quiet and studious in her youth, Shirai's years of exposure to her mother's endless criticism combined with being written off as gloomy, serious, and plain by her peers without a second thought caused her to become embittered, asocial, and resentful of the world. Not only did this result in her adopting a highly disciplined, traditional, no-nonsense teaching style — one which clashes horribly with the enthusiastic, positive-thinking approach of Daisuke Aoki and causes them to butt heads on just about every topic imaginable — it all but eliminated any semblance of a social life as she has neither had sexual relations nor even dated anyone despite being nearly thirty years old, the former of which remains a tremendous bone of contention for her.

After exploding at Kuro Kagami for mocking Aoki for being a virgin (unintentionally betraying her own status as one to him in the process), Shirai was completely taken aback when the young girl approached her a short time later, offering her not only fashion tips and suggestions on improving her appearance but also professing great interest in being her friend by dubbing her "Shiro-chan". Though annoyed by the sudden, newfound, and unwanted attention she was receiving, she nevertheless remained remarkably tolerant of it and restrained by her standards. When asked for advice by Kuro on what the best approach for dealing with her unrequited love for Rin would be, Shirai hurt the young girl's feelings by unintentionally implying that her lesbian attraction was somehow less than normal. Only later, following a series of unrelated events in her own personal life, did she realize the full extent of what she had inadvertently done and what a poor judge of normalcy she, of all people, was.

Seeking to make amends by wearing a white hairpin previously given to her as a gift by Kuro, Shirai was privately flattered when her admirer demonstrated her acceptance of the apologetic gesture by beating up a male student who made a disparaging remark about her appearance, the event marking something of a turning point for her. Though hardly transformed into a social animal by the experience, Shirai has shown signs of softening somewhat, once to the extent where she broke her own rules concerning physical contact between students and teachers by offering a comforting hug to a heartbroken Kuro at an athletic meet.She has also recently started rethinking certain aspects of her life — most notably how much she has become like the mother she so loathes — and repaid Hōin's favor of getting her out of a teacher's workshop by helping her deal with the belligerent mother in charge of the local parent-teacher association, achieving some closure and inner peace of her own in the process. It is (very slightly) hinted that she is developing a sort of attraction towards Aoki, after a very awkward moment between them in a train they inadvertently were on together, although she continuously denies this.