Moaning Myrtle....

Moaning Myrtle is a ghost who haunts the second floor girls' lavatory at Hogwarts. True to her nickname, she has a tendency to sob, whine, wail and complain, especially when death is referred. Her constant moping and wailing causes plumbing problems in the lavatory she haunts. It was revealed by Rowling on her website that Myrtle was in Ravenclaw House.

In Chamber of Secrets, it is established that the character is the ghost of a Muggle-born witch who died while a student at Hogwarts, fifty years before the events in the book. It is revealed that Myrtle was hiding in the second floor girls' lavatory to elude Olive Hornby, a classmate of hers who perpetually tormented her about her glasses, when the Chamber of Secrets was opened and the basilisk emerged and killed her. Tom Riddle used her death to create his second Horcrux: his diary. After death, Myrtle haunted Olive everywhere she went, until Olive complained to the Ministry of Magic, who ordered Myrtle to return to Hogwarts. Ever since then, Myrtle has sulkily haunted the same lavatory where she died.

Myrtle also helps Harry with his second task in the Triwizard Tournament, in Harry Potter and the Goblet of Fire. Myrtle tells Harry how to solve the puzzle of the golden egg that he retrieved in the first task, by opening the egg underwater. She is shown to be a lot less miserable, and enjoys having Harry briefly to herself to boss around. In Harry Potter and the Half-Blood Prince, she appears to comfort Draco, who is worried about the task given to him by Voldemort. After Harry injures Malfoy using Sectumsempra, Myrtle is not hesitant to spread the news throughout the school, screaming that Harry had murdered her.