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.