Moaning Myrtle

Age: Killed by a Basilisk under the command of Tom Riddle when in her early teens.
House: Ravenclaw
Distinguishing Characteristics: Thick glasses, lank hair, and a seriously unhappy demeanor. Now deceased, she is a ghost who can usually be found in the girls' toilet.
Heritage: Muggle-born
First Mention: Chapter Eight of Chamber Of Secrets.
General: In his fifth year at Hogwarts, Tom Riddle, the heir of Slytherin and prefect who was later to become Lord Voldemort, discovered the entrance to the Chamber Of Secrets. This was a chamber created by Salazar Slytherin (one of the Hogwarts founders) beneath the school, containing a Basilisk who could be commanded by Slytherin or any of his heirs to attack and kill Muggle-born students. Riddle released this monster only once; a Muggle-born girl was killed in one of the bathrooms. Her name was Myrtle.
Myrtle, upon becoming dead, decided to stay in the toilets rather than moving on. She was determined to haunt the girls who had bullied her in school. This is characteristic of Myrtle in general - the schoolgirl's pettiness and abysmal self-esteem carried on into the afterlife. She is extremely sensitive with regards to any comments involving her personal appearance or the fact that she is no longer alive.
Myrtle first encounters Harry Potter in his second year at Hogwarts when he and his two closest friends use Myrtle's toilet for illicit potion-brewing. (The setting is ideal as nobody uses Myrtle's toilet.) Ironically, Harry, Ron, and Hermione are brewing the potion in order to find out more about the Chamber of Secrets, not realizing that the answers they needed were there in the toilet all along.
Eventually they do realize that Myrtle is the Muggle-born girl who was murdered by the Basilisk, and she helps them to find the entrance to the Chamber, leading to the rescue of Ron's younger sister Ginny.
This is not the last time Myrtle helps Harry. Two years later, when he is entered in the Triwizard Tournament, she aids him with his second task - both by helping him solve his clue, and by pointing him in the right direction during the underwater task in the Great Lake.
While Harry does not see Myrtle regularly, she always has a habit of turning up and providing help when it is needed. This may be a result of her schoolgirlish infatuation with Harry, a genuine desire to help, or pure luck. Either way, she has so far proved an asset to Harry and his friends.
However, this generosity in recent times has been extended further than merely to the Trio - in Harry's sixth year, Myrtle has also been dwelling in a boy's toilet, acting as a sympathetic ear for Draco Malfoy with regards to his recruitment into the Death Eaters and the impossible task given him by Voldemort. Following a confrontation between Harry and Draco which resulted in Draco's serious injury, it was Myrtle who raised the alarm. Therefore it is unknown whether or not Myrtle will prove to be such a help to Harry in the future, although, should he remain away from Hogwarts, their paths may not cross.