Koumura is the elderly classical literature teacher of Tomoya's school. He is mild-mannered, polite, and caring. Koumura was Tomoya's and Youhei's homeroom teacher in his first year and he is the last remaining teacher who hasn't been assigned as an adviser for a club. Tomoya treats Koumura disrespectfully but Koumura doesn't seem to mind.
Koumura is currently in his last year as a teacher. He is a good friend of Kouko during their time as teachers.
6 years ago, Koumura taught at the neighboring engineering high school which is famous for the number of delinquents studying there. Koumura used to be rather active and he would counsel the students into becoming better people. Many students left their delinquent ways due to the effort of Koumura.
Eventually, Koumura transferred to Tomoya's high school to look for a less demanding atmosphere. Koumura could focus on teaching instead.
Koumura displayed his prowess in martial arts and became the last person to extend Tomoyo's kick combo on Youhei.
Sunohara Siblings' path
Koumura recognized the similarity between Tomoya and Youhei and he set them up to become friends during their first year.
Tomoya requested Koumura to organize Kouko's wedding ceremony at the school.
Kouko visited the Furukawa bakery and sent Tomoya to meet with Koumura. She explained Koumura's past. Tomoya realized that Koumura had been helping him and Youhei to graduate by having them befriend each other. At the graduation ceremony in Spring, Tomoya and Youhei approached Koumura. Koumura explained that he considers them his children and they thanked him.
Tomoya and Nagisa requested to meet Koumura, the adviser of the drama club of the previous year. Koumura sent them to meet Nishina who had also requested him to be the adviser of the choir club. Nagisa sympathized with Nishina's story, thus she decided to give up.
After the basketball match, Tomoya and Nagisa met Nishina at the drama club room. Nishina suggested sharing Koumura as their club advisers. Tomoya and Youhei volunteered to join the club to allow the drama club to be officially formed. Nishina invited Nagisa to participate in the School Founder's Day.
Koumura oversaw Nagisa as she prepares for her solo drama performance.
Koumura headed Nagisa's mock graduation ceremony.