Mimi Usa is a quiet and studious girl who is part of the main group of friends in the series, alongside Rin Kokonoe and Kuro Kagami. She is physically advanced for her age in terms of development, which initially sets her apart from her classmates. Background and early story focus on her experience with bullying by her teacher before Daisuke Aoki took over the class. She quit attending school to protect herself, and her friend Rin retaliated by writing threatening notes to that teacher, which eventually led him to resign. Aoki, the new homeroom teacher, works to convince Mimi to return to school, which she does. Her personality is characterized by shyness and a thoughtful, reserved nature. She is serious about her studies and prioritizes her education. Her motivations primarily revolve around her friendships and her quiet feelings for Reiji Kokonoe, Rin's guardian, toward whom she harbors a secret crush that she eventually acts on in the later parts of the manga. In terms of role in the story, Mimi serves as a calm and grounding counterpoint to the more outgoing and impulsive personalities of Rin and Kuro. Her key relationships include her close friendship with Rin and Kuro, her trust in Aoki after he helps her return to school, and her romantic interest in Reiji. Her development over the course of the series shows her becoming more confident and assertive, especially in pursuing her feelings for Reiji. Notable abilities include her academic diligence and her physical growth, which causes her to be perceived as mature for her age. In the OVA Kodomo no Jikan Kodomo no Natsu Jikan, Mimi joins Rin, Kuro, and their teachers on a summer trip to Aoki's hometown, contributing to the group dynamics of the summer vacation story.