Reason a demon may come out of a mirror?

I am writing a short comedic story for English and I am stuck. I have a girl getting ready for a party, and as she's looking in a mirror, a demon or a ghost comes out and scares her. I have not decided yet which he will be. I have ideas as to why a ghost would come out and haunt her, but a demon seems more plausible for furthering the story. I am not sure why he would arrive and stay with her though. Any ideas?