Moaning Myrtle is a ghost found in the Harry Potter novels. She makes her appearance in the second book, Harry Potter and the Chamber of Secrets. She haunts one of the girls' washrooms of Hogwarts, the wizarding school that Harry and his friends attend.
*If you haven't read further than Harry Potter and the Chamber of Secrets, spoilers ahead*
Moaning Myrtle was, as Harry, Ron and Hermione discovered, the student who had been killed when the Chamber of Secrets had been opened fifty years earlier. Her killer, the Basilisk still remained in the Chamber of Secrets, until Harry killed it with the help of the sword that had belonged to Godric Gryffindor, the founder of Gryffindor house. He pulled this sword from the magical Sorting Hat. The Basilisk had been directed by Tom Riddle, the young manisfestation of the evil Lord Voldemort, through his diary communications to Ginny Weasley.
After formulating all sorts of plans in Moaning Myrtle's washroom on how to discover the Chamber of Secrets, Harry and company discovered that the key was right in there with them the whole time- Moaning Myrtle.
Moaning Myrtle believes that everyone enjoys teasing her and wishes her ill. Therefore, she spends much of her time moping and crying about her misfortune.