Aoi Koshimizu serves as the school nurse at the high school attended by the main characters. She is a young woman with semi-long blonde hair and a tendency to wear a white lab coat that complements her appearance. Her facial features include gentle, drooping eyes and a distinctive duck-like pout to her lips, which contribute to a laid-back and approachable demeanor that has made her quite popular among the male student body.

Beneath this calm exterior, however, Aoi possesses a markedly predatory and uninhibited personality. She has a well-documented fondness for raunchy jokes and salacious topics, and she displays little restraint in expressing her open attitude toward sexuality. Her behavior is often forward and flirtatious, leading to rumors that she frequently seduces the male students who come to visit the infirmary. This uninhibited nature extends to her questionable teaching methods; for instance, she has been known to instruct students that the consciousness check in artificial respiration should be replaced by asking the patient if they like the person administering the aid.

Aoi shares a deep and long-standing friendship with Ichika Omori, who serves as the homeroom teacher for the main cast. Their bond dates back to their own high school days, and they maintain a close relationship characterized by the affectionate nicknames Icchan for Ichika and Aoi for herself. During their youth, Aoi was a supportive presence for Ichika, particularly during a period when her friend was experiencing unrequited love. In the present, she continues to involve Ichika in her antics, frequently dragging her into conversations laced with innuendo or collaborating with her on explicit jokes.

Her role in the story is primarily that of a supportive adult figure whose over-the-top and candid approach to sexual matters serves as a source of comedy. She interacts with the adolescent characters by teasing them and providing a very direct, if unconventional, form of guidance. While she is deeply involved in the health management of the students, her personal brand of sexual education is often more about her own enjoyment than formal instruction. There is no significant character development noted for her, as she remains consistently forthright and mischievous throughout the narrative. Her most notable ability is her complete lack of inhibition and her talent for making others uncomfortable with her blunt, sexually charged comments.