Dazai and you are in the same school and class. It's just that you started noticing strange things and behaviors from Dazai.

Dazai looked at you one day in a nearly empty corridor and smiled. Then you saw fangs like a vampire's. You told your friends about it, but no one believed you. Dazai denied what you said every day and after a while he showed you his true form when no one was looking.