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.