Peeves
Distinguishing Characteristics: Peeves is small in stature but has a wide face and mouth and beady eyes. He usually wears a hat covered with bells, brightly-colored tie and clothes, and slippers with long, curly toes. He specializes in making his presence known; he never walks but floats and bobs above crowds, making all kinds of racket as he goes.
First Mention: Sorcerer's/Philosopher's Stone, Chapter 7
General: The word poltergeist means noisy ghost. Peeves is noisy, but he's not a ghost. Poltergeists are thought to be manifestations of psychokinetic energy, not the imprints of deceased people. The Hogwarts ghosts themselves admit (gladly, no doubt) Peeves is not one of them. Theorists believe poltergeists are caused by the subconscious activity of particularly stressed people. With the stress Hogwarts students have undergone for centuries, it's no wonder Peeves is such a well-developed psychokinetic mischief-maker.
Unlike the ghosts, Peeves can touch objects and move them. He hurls water balloons, ink bottles, sticks, chairs, chalk, rubbish bins, even tarantulas. While ghosts float through walls, Peeves can turn doorknobs to open doors, or he can burst through blackboards. He can make himself invisible, but he always makes himself heard, blowing raspberries, cursing, and banging things around. An avid musician, Peeves can come up with an obscene and/or insulting ditty for any occasion, from Christmas to petrifications - sometimes even choreographed. When there aren't many students in, he bounces around the trophy room (Prisoner of Azkaban and Goblet of Fire).
Peeves' archenemies are Filch and, later, Umbridge - he revels in the exasperation of everyone, but particularly these two people. Perhaps their respective bitterness and blind ambition are more antithetical to Peeves' sense of fun than anything else at Hogwarts. He torments students and shows as little respect as possible to professors. When faced with a genuine authority figure such as Dumbledore, Peeves adapts superficial politeness. Only the Bloody Baron can control Peeves. From Peeves' reaction in Philosopher's Stone to Harry pretending to be the Slytherin ghost, we see Peeves treat him with respect. He often speaks with poor grammar and refers to himself in the third person, including when he addresses the Baron, but he obliges him gracefully and even humbly. No one ever claims Peeves fears the Bloody Baron, though; he may not fear anything.
We know he does have desires, however. Both Philosopher's Stone and Goblet of Fire refer to his attempts to gain admission to the start-of-term feasts. He always tries to cause the most panic and chaos possible or at least he tries to witness it. In the middle of Philosopher's Stone, he tells on Hermione, Harry, Ron, and Neville for being out of bed, but refuses to tell Filch which way they've gone. Thus he causes the students to fear expulsion and enrages the caretaker. He composes various rhymes which question Harry's state of mind, and calls Lupin "loony Lupin," probably a play on words regarding Lupin's connection to the lunar cycle - he hits people where they are most vulnerable. Obviously with all his bouncing around, Peeves is attuned to the inner struggles of the castle's inhabitants. During her stay at Hogwarts, Fleur Delacour derides the staff for permitting Peeves to stay. It's easy to question Dumbledore's judgment in allowing a creature who grabs students' noses, wakes them at 4:30 AM, and even tries to strangle them with tinsel, to remain in the castle. But can he really control Peeves when the students themselves may have inadvertently created him? And if Peeves were "expelled," couldn't his knowledge fall into the wrong hands?