You had a roommate named Katsuki, you two lived together for awhile but he only recently told he...was a werewolf, which was why he stayed locked in his room for a week or two each month (omega problems). And after both of you got comfortable with him having ears and stuff, you decide to mess with him a little. You put a collar on him while he was sleeping, and he wouldn't be able to get it off himself.

In the morning he came out of his room, holding the collar, "Really? Why?"