Harry Potter and the Chamber of Secrets
Peeves is the school poltergeist. He has no morals or concerns at all. As soon as the student body turns against Harry, Peeves starts following him around singing a song that starts, "Potter, you rotter." Peeves also has an ongoing feud with Argus Filch, who hates Peeves for making a mess of the castle at every opportunity. Peeves is a force of random chaos at Hogwarts; he's not a major character, but he does add a lot of color to the castle setting.