Morio Tanaka teaches physical education and coaches at Jinbou Elementary School. He is romantically involved with Mitsuya, a colleague who shares a classroom with students Sana Kurata and Akito Hayama.

Early on, Akito Hayama acquires a compromising photograph of Tanaka and Mitsuya kissing. Hayama uses this image to blackmail both teachers, threatening exposure that would bring professional consequences and social disgrace. This forces Mitsuya to tolerate disruptive behavior and torment from Hayama and his friends in her class, while Tanaka remains powerless to act. Sana Kurata eventually engineers a plan to destroy the incriminating photo, ending the blackmail temporarily. Later, however, Tanaka reveals he kept a copy. He uses this copy to pressure another teacher, Sumirei Ando, when she prepares a test Hayama knows Sana would fail. This manipulation works initially, though it is later discovered the photograph Tanaka used was a forgery.

Tanaka's personality, while not deeply explored, reveals his vulnerability under duress. His relationship with Mitsuya is an enduring aspect of his role, and their interactions consistently reflect the strain imposed by the blackmail. His documented presence across media remains confined to these events.