Muggles' Guide to Harry Potter/Characters/Moaning Myrtle

Overview
Moaning Myrtle, born as Myrtle Elizabeth Warren, was a former Ravenclaw student (now a ghost) who haunts the girl's bathroom on the first floor (called the second floor in the US editions of the book) of Hogwarts School of Witchcraft and Wizardry.

Chamber of Secrets
We meet Myrtle at Nearly Headless Nick's Deathday party. She seems a bit depressed, as many ghosts do; but Peeves takes the opportunity to mock her to the point that she runs off, crying. Apparently as she returns to her bathroom, she turns on all the taps.

Later, passing the spot where Mrs. Norris, Filch's cat, was Petrified, Harry, Ron, and Hermione realize that they are passing Myrtle's bathroom. They stop in to talk to her, but she is not terribly communicative. They are caught exiting from the bathroom by Percy, who threatens them with a House points penalty.

Because Myrtle's bathroom is such an unwelcoming place, Hermione suggests it would be perfect for their use as they brew up a batch of Polyjuice Potion. They do this starting at about the middle of November. Myrtle doesn't seem to mind this activity much, usually hiding underwater somewhere and gurgling at them. When the time comes for them to use the potion, Hermione finds that she has made a mistake and used a cat hair; Myrtle seems very amused that Hermione has turned partway into a cat and is now stuck there. She finds Hermione's tail especially amusing.

Later, Harry and Ron find Myrtle's bathroom flooded; apparently she has become upset because someone threw a book at her while she was resting in the U-bend, thinking about death. In her anger she has turned on all the water in the bathroom. Harry recovers the book, it turns out to be a diary, originally owned by one T. M. Riddle fifty years ago.

A clue in that diary leads Harry and Ron to Hagrid, who in turn sends them to Aragog. Aragog, in turn, mentions that the last time the Chamber was opened, a girl died in a bathroom. It suddenly occurs to Harry that this might be Moaning Myrtle.

On their way to visit Myrtle for confirmation, Harry and Ron are caught in the halls by Professor McGonagall. Thinking up a reason for their being in the halls, Harry hits upon the idea of saying that they are trying to visit Hermione in the Hospital wing, as she has been Petrified. While visiting Hermione, they find a scrap of paper that she has torn out of a book which describes Basilisks; this tells them what the monster is.

Finally, with Professor Lockhart in tow, they return to Myrtle's bathroom. Myrtle confirms that it was she who had died the last time the Chamber was opened, and what she remembers matches the effects of a Basilisk; Harry figures out how to open the Chamber, and with Ron and Lockhart, he descends. When he returns, Myrtle suggests that if he had died down there, he would have been welcome to share her bathroom; Ron teases that Harry's got a girlfriend.

Goblet of Fire
Harry is given a hint to the Golden Egg clue by Cedric Diggory, and as a result ends up taking the egg to the Prefects' washroom. Moaning Myrtle joins him there, appearing out of one of the faucets, to his dismay – Harry tries to hide his nakedness under the bubbles; Myrtle after all is a girl. Myrtle guides Harry towards what Cedric did to his Golden Egg: opening it underwater and listening to it there. While Harry is thinking about the meaning of this clue, Myrtle is telling how she became a ghost; Harry largely ignores her. When Harry muses how he is going to breathe underwater, Myrtle becomes affronted at being reminded that she doesn't breathe any more.

During the Second Task, Harry, swimming deep in the lake, is again accosted by Moaning Myrtle. Harry is surprised that she is able to get to the lake; she says that sometimes she is sent to the lake by surprise when someone flushes her toilet unexpectedly. She points Harry in the direction of the Merpeople's settlement under the lake.

Half-Blood Prince
At one point, Harry and Ron step into a bathroom to allow Ron to escape the clutches of Lavender Brown. Myrtle appears and is disappointed, saying that Harry had promised to visit her and never did, and that she was hoping that it was the other, sad boy who was coming in. She says that he is worried because he has a task and he has been working at it for a long time, and is not succeeding. Ron manages to insult her by mentioning that she is dead, and she leaves through the plumbing, crying.

Harry, looking at the Marauder's Map, sees Draco Malfoy in a nearby bathroom with Myrtle. Looking in at them, he sees that Draco is crying. Draco catches sight of him in the mirror, and immediately starts flinging curses at him. Harry retaliates. Draco starts to invoke the Cruciatus curse, but Harry stops him with the Sectumsempra curse, one which he has read but never tried to use. This curse makes Harry's wand act like a very long knife, making great cuts appear in Draco's face and torso. Myrtle starts screaming, saying there was "murder in the bathroom"; Severus Snape appears and manages to heal Draco.

Weaknesses
Myrtle is offended very easily, at least initially. Any reminder that she is dead will send her off in gouts of tears. When Tom Riddle's diary was thrown into the commode where she happened to be invisibly resting, and it happened to fall through her, this enraged her to the point that she caused all the toilets and taps to effectively erupt, sending water all through her bathroom and out into the hall. It is in part this hypersensitivity that has left her with the name Moaning Myrtle, because in fact that does seem to be what she does most of the time.

Relationships with Other Characters
When Harry returns from the Chamber of Secrets, she suggests that if he had died down there, Harry would have been welcome to share her bathroom. Additionally, she helps him a couple of times before and during the Second Task of the Triwizard Tournament. This would seem to indicate that she has a little bit of a crush on Harry. However, when Ron and Harry use a bathroom to hide from Lavender Brown in Harry Potter and the Half-Blood Prince, Myrtle seems disappointed to see them, which would indicate that any romantic interest she has in Harry has been replaced by an interest in another boy, who turns out to be Draco Malfoy.

Analysis
Most unusually for a ghost, it seems Myrtle does have some control over physical matter; apparently her haunting of the bathroom has rendered it unusable, mainly because she keeps flooding the place when she gets depressed. This is useful to the story in Harry Potter and the Chamber of Secrets in two respects: Mrs. Norris is Petrified (in the chapter The Deathday Party), rather than being killed, because she sees the reflection of the Basilisk rather than seeing the Basilisk directly. The image is reflected in the water that Myrtle has caused to flood out of the bathroom because she was so upset at the insults lobbed at her by Peeves at Nearly Headless Nick's Deathday party. Additionally, it is Myrtle's unwelcoming presence in the washroom that allows Hermione the time and space to make Polyjuice Potion; nobody willingly goes in there more than once, after being subjected to Myrtle's aggressive depression.

Myrtle's affinity for water also allows her to help Harry with the Second Task in Harry Potter and the Goblet of Fire; as the second task involves the Merpeople under the lake, Myrtle is instrumental in allowing Harry to solve the riddle surrounding the task, and also points him in the right direction when he is looking for the Merpeople during the task proper. Myrtle says that she sometimes ends up in the lake when someone flushes a toilet in her bathroom and she's not expecting it, which may lead us to wonder about Wizarding sanitation practices.

As mentioned, Myrtle is always sad and is apparently still hypersensitive about death. It is true that she had died only fifty years before our story opens, and so could have expected, in the normal run of affairs, to still be alive; it is possible that a ghost will be especially sensitive to thoughts of death until they reach an age at which they would have died normally. As far as her perpetual sadness goes, we must remember that at the time she died, she had been driven to tears by the cruelty of another girl and was hiding in the washroom. From what Nearly Headless Nick tells us in Harry Potter and the Order of the Phoenix regarding ghostly existence, it is entirely possible that Myrtle is sad as a ghost purely because she was so sad when she died.