Moaning Myrtle

Moaning Myrtle is introduced in Harry Potter and the Chamber of Secrets as a ghost who haunts the second floor girls' lavatory at Hogwarts. Unlike the other Hogwarts ghosts, Myrtle is not very sociable. True to her nickname, she has a tendency to sob, whine, wail and complain, especially when death is referred to. Hermione aptly points out, "She's a little sensitive." Her constant moping and wailing causes plumbing problems in the lavatory, which she haunts. It was revealed by Rowling on her website that Myrtle was in Ravenclaw.[15]
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. The books indicate that Myrtle was often bullied during her years at Hogwarts, leading her to become extremely depressed and often retreat to the second floor girls' lavatory to weep and cry her depression to tears. It is revealed through the events in the book that Myrtle was hiding there to elude Olive Hornby, a classmate of hers who perpetually tormented her with teasing, when the Chamber was opened and the basilisk emerged and killed her. Tom Riddle used her death to create his very first Horcrux: his diary. It is also worth mentioning that Moaning Myrtle was Riddle's first known victim. 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 haunted the same lavatory where she died.
Myrtle's role in Harry Potter and the Chamber of Secrets is to help Harry, Ron and Hermione find out about the monster inside the Chamber of Secrets. She is quite disappointed that Harry survives his encounter with Slytherin's monster, as she would have liked him to join her in death. 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. In Goblet of Fire, Myrtle is a lot less miserable, and enjoys, it seems, having Harry briefly to herself to boss around. In Harry Potter and the Half-Blood Prince, she appears to comfort Draco Malfoy, 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 him.