Toshio Koumura is a language teacher at Tomoya's school and was his homeroom teacher during his first year. Despite his age, he once had a vigorous past filled with enthusiasm for teaching. Three years ago, he was friends with Kouko Ibuki.
Five years ago, Koumura was a strict teacher at an engineering school known for its delinquents. He successfully reformed many of these students, but at Tomoya's current school, his skills are less in demand due to the school's prestigious reputation. He sees Tomoya and Youhei as unusual cases and decides to help guide them towards improvement; he is the one who arranged for Tomoya and Youhei to meet.
Koumura retires a year before the end of the school segment of the story. In After Story, he returns to organize a mock graduation ceremony intended for Nagisa, set up by Tomoya.
