Moaning Myrtle is a ghost who haunts a girls’ toilet. She is squat, with a glum face hidden behind lank hair and thick glasses. She has a habit of throwing tantrums and flooding the toilet, which has rendered it out of use all year.
At Nearly Headless Nick’s Deathday Party, Peeves overhears Harry, Ron and Hermione talking about Myrtle and calls her over. Myrtle insists that they were making fun of her and flees the dungeon, sobbing.
The attack on Mrs Norris happens right outside Myrtle’s bathroom; Harry, Ron and Hermione enter it to ask her questions a few days later. Myrtle says she wasn’t paying attention the night of the attack because she was too busy trying to kill herself – until she remembered that she is already dead.
While Harry, Ron and Hermione are taking the Polyjuice Potion, Myrtle is gurgling morosely in the end cubicle. Myrtle looks unusually happy at the prospect of Harry and Ron witnessing Hermione's botched Polyjuice transformation.
Early in the second term of the year, Myrtle is found crying even louder than usual. She tells Harry and Ron it is because someone has thrown a book through her head. She directs them to the diary, but says she does not know who threw it.
After Aragog reveals that the student who was killed by the monster of Slytherin died in a bathroom, Harry realises that Myrtle was probably the victim. When Harry asks her how she died, Myrtle says she was crying in the bathroom, heard a boy’s voice and saw a pair of large yellow eyes. She died instantly, and returned to haunt Olive Hornby, a girl who used to tease her.
Moaning Myrtle reveals herself to Harry in the Prefects' bathroom after he has entered the bath. She sits cross-legged on top of one of the taps, and tells Harry he should try putting the golden egg in the water. She tells Harry that she closed her eyes when he got in the bath, but is upset that he hasn't been to see her in a while in the out-of-order girls' bathroom she haunts. She appears to be thoroughly enjoying herself as she bosses Harry around, instructing him to fetch the egg and open it underwater, and to put his head under as well. Myrtle says that she saw Cedric Diggory doing the same thing with the egg in the Prefects' bathroom previously, revealing that she sometimes sneaks up to the Prefects' bathroom and watches them take baths. She says that she has never come out to speak to anyone before Harry arrived.
When Harrys asks what creatures live in the lake at Hogwarts, she admits there are all sorts of creatures as well as the giant squid. She sometimes goes down there, mostly when someone flushes her toilet unexpectedly. She is impressed when Harry realises there are merpeople in the lake, but takes offence when Harry asks her how he is meant to breathe underwater: she thinks it insensitive to ask when she can't breathe herself. She is sad when Harry leaves the bathroom, and asks him to visit her in her bathroom sometime soon.
Moaning Myrtle enters the lake at Hogwarts during the second task. She asks Harry how he is getting on, scaring him slightly, and floats lazily in front of him in the water. She points Harry in the direction of the merpeople, but tells him she won't come with him as the merpeople always chase her when she gets too close.