Michiru Onigawara is the school nurse at Sakuramori High School and serves as the adviser of the Second Voluntary Club. She is an adult woman with long blue hair and purple eyes, often seen in a lab coat or nurse uniform. Her background as a medical professional gives her a veneer of authority, but her true nature leans toward sadism. She enjoys making people—especially attractive girls—perform cosplay and taking photographs of them in the outfits she designs. She holds leverage over various school officials by knowing their weaknesses, which allows her to manage troublesome situations with relative ease.

Personality-wise, Michiru is dominant and enjoys exerting control over others, often threatening students with physical harm or embarrassment to get her way. Despite this, she displays a caring side toward those she is close to, particularly Mio Isurugi, whom she treats almost like a younger sister. Mio calls her "Michiru-nee" and the two share a close bond. Michiru also shows a playful, mischievous streak, frequently teasing Taro Sado about his masochistic tendencies and placing him in uncomfortable situations as a form of "treatment." She does not fully recognize her own sadistic impulses, instead rationalizing her behavior as necessary for the club's activities.

Her primary motivation appears to be the enjoyment she derives from her hobbies—cosplay photography and mild domination—rather than any grand goal. Within the story, she acts as a recurring supporting figure who both enables and complicates the main characters' efforts. Her role as club adviser gives her a reason to involve herself in the antics of the Second Voluntary Club, and she often uses her position to push events in chaotic directions. She does not undergo significant character development over the series but remains a consistent source of comic relief and conflict.

Key relationships include her mentorship of Mio Isurugi, whom she encourages in her self-proclaimed goddess persona, and her adversarial yet cooperative dynamic with Taro Sado, who becomes a frequent target of her sadistic games. She also interacts with other club members such as Arashiko Yuno, though less prominently.

Her notable abilities are not supernatural but social and practical: she is skilled at manipulation, costume design, and using her medical authority to pressure students. Her sadistic tendencies, combined with her creativity and institutional power, make her a uniquely disruptive presence in the school setting.