Mitsuya was Sana, Hayama, Tsuyoshi, Hisae, Aya, Mami, and Gomi's, amon other's, 6th grade teacher at Jinbou Elementary. Early in the year, no teaching could be done due to Hayama blackmailing her and her boyfriend/co-worker Tanaka, forcing her to allow him and his lackeys to wreak havoc in the classroom. When Sana blackmails Hayama, she manages to destroy the evidence of her relationship and bring order back to the classroom.
She is seen as being fairly weak, and cries regularly as she suffers through Hayama's actions. She pulls through when the eivdence is gone, however. She sides with the girls when the classroom is split again, forcing the boys upon Tanaka.