Toshio Koumura is a language teacher at Tomoya's school and Tomoya's home room teacher during his first year. His age conceals his once vigorous past marked by his enthusiasm in teaching. He was friends with Kouko Ibuki three years ago.

Five years ago, he was a strict teacher in an engineering school being well known for delinquents. He changed a lot of delinquents in the engineering school for the better, but at Tomoya's school, his skill is not needed due to the prestigious image of the school. He finds Tomoya and Youhei to be odd ones and decides to change them for the better; he is the one who arranged Tomoya and Youhei to meet.

He retires one year later, prior to the end of the school segment of the story. In After Story, he returns and organizes the fake graduating ceremony intended for Nagisa, arranged by Tomoya.