Otsuki is a teacher at Karukozaka High School known for saying "there's no scientific proof that...", who quickly vowed his loyalty to Hazama after the latter throws the school into the Expanse. However, Hazama himself hates Otsuki since he only shows loyalty in order to gain favor and has only failed in stopping the protagonist.
The protagonist first confronts Otsuki after defeating either Faunus or Melusine at the school. Each time he is defeated, he rebuilds himself in order to take the heroes down the next time. His second fight takes place in the World of Gluttony, where he drops the Micro Plasma needed to enter Orcus' body and defeat Parasite.
In Yumi and Reiko's routes, he challenges the protagonist for a third time after Lilith's defeat in the World of Envy. In the World of Greed, he steals the Ring of Kindness from the protagonist and tells them to come to his lab, where they have their final battle.
In Charlie's route, Hazama himself asks the protagonist to kill Otsuki in the World of Wrath. Otsuki refuses to accept that Hazama dislikes him and says it's a lie the protagonist made up to confuse him. He escapes, but later reappears in front of the portal leading to Hazama for a final battle.
Otsuki is obscurely referenced by a soul in the Medical Center Lobby in Shinjuku Medical Center upon its completion. He says "What? Ms. Takao? Dr. Ostuki? There's no scientific proof that such doctors are in this hospital..."
Early in the game, speaking to a Pixie will result in it referring to Otsuki as a "pervert" who "loves the Demon Emperor," implying an attraction towards Hazama; despite the fact that Hazama is an underaged teenager.