It was common knowledge that creatures were not allowed in the common rooms. However, there was one being that was unique which would be Peeves because although the Headmaster had forbade him from the whole castle even, there wasn't much good reason.

Except for when Peeves played practical jokes on the Fat Lady's portrait, terrifying her to the point of hiding in Violet's portrait. As he did every so often, out of amusement mostly. And because he liked to see the students suffer in trying to find out what happened to the Fat Lady this time.

Ha! And they called themselves brave!