Kosaka is a staff member at the junior high school attended by Shizuku Tsukishima. She serves as the school’s health room teacher, a position that combines elements of a nurse and a counselor. Her duties include tending to students who are sick or injured and offering emotional support to those facing personal difficulties. She maintains a close and approachable relationship with Shizuku and her friends, often allowing them to eat lunch in her office. This open demeanor makes her a trusted adult figure within the school setting. Her motivation centers on the well-being of her students, and she provides a safe space where they can relax or seek help. In the story, her role is that of a steady, supportive presence rather than a driving force in the plot. She does not undergo a major personal transformation, but her consistent guidance reinforces the theme of community and quiet mentorship. Her key relationships are with Shizuku and her close circle, to whom she offers both practical care and a listening ear. Notable abilities include her knowledge of basic health care and her skill in counseling adolescents, which she applies in her daily work.