"It's awful trying to have a pee with her wailing at you--"
     -- Hermione Granger

"D'you think I don't know what people call me behind my back? Fat Myrtle! Ugly Myrtle! Miserable, moaning, moping Myrtle!"
     -- Moaning Myrtle

Moaning Myrtle is a ghost who haunts a toilet in a girls' bathroom on the second floor [1] of Hogwarts castle. Myrtle was a student at Hogwarts in 1942 when she was killed by the Basilisk from the Chamber of Secrets. She had been hiding in that bathroom because she was being teased by Olive Hornby. Then she heard someone come in and heard them talking. She realized it was a boy, so she opened her stall to tell him to get out of the girls' bathroom, only to find herself face to face with the Basilisk. Myrtle decided to haunt Olive Hornby to make her pay for teasing her so  much. She followed Olive around until the Ministry had to step in and stop her. So Myrtle went back to her toilet and moaned and cried and splashed water out whenever she felt particularly miserable.

Myrtle is a squat ghost with lank hair, pimples, and thick glasses. She seldom smiles and takes offense at the least excuse, crying rivers of tears and wailing. Peeves enjoyes teasing her.

Myrtle has lately taken quite a shine to Harry Potter. When he nearly died going into the Chamber of Secrets, she told him that if he had died, he could share her toilet (CS17). During the Triwizard Tournament, Myrtle helped Harry figure out the egg clue (while admitting that she sometimes came into the Prefects' Bathroom to spy on bathing students) (GF. She also turned up in the Lake and directed him toward the Merfolk village. (GF

1 - Although Hermione says at the Deathday Party that Myrtle haunts a bathroom on the first floor, she is mistaken. When Harry, Ron, and Hermione run upstairs following the voice, they run up to the second floor (and the book clearly states that they are on the second floor) to where they see the cat hanging outside the bathroom.