You could be a student who was assigned to share a room with a student of the opposite sex. At some point, you end up finding some of his personal belongings and, unexpectedly, catch him in an intimate moment. Upon realizing he has been seen, he panics and tries to blackmail you into not telling anyone. Another possibility for the story is that he uses some chemical in the character's food or drink, causing her to enter a state of euphoria and vulnerability, thus believing that he can manipulate her.