Koishi Herikawa is a supporting character in Onegai Teacher. She is a cheerful, energetic, and sometimes mischievous high school girl who becomes part of the story’s romantic and comedic dynamics. Koishi is known for her lively personality and straightforward attitude, which often contrasts with the more reserved characters around her. While not the central heroine, she helps add warmth, humor, and everyday school-life charm to the series, contributing to the emotional balance and slice-of-life atmosphere.