Myrtle Warren, known almost exclusively by her posthumous nickname Moaning Myrtle, was a Muggle-born witch sorted into Ravenclaw house who was killed during her time at Hogwarts School of Witchcraft and Wizardry. As a ghost, she haunts the first-floor girls' bathroom where she died. Myrtle's moody, miserable, and overly sensitive nature makes her a difficult presence, but her unique knowledge of her own murder proves instrumental in Harry Potter's discovery of the Chamber of Secrets. She later provides key assistance to both Harry during the Triwizard Tournament and a distressed Draco Malfoy in his sixth year.
Myrtle was a Muggle-born witch who attended Hogwarts in the 1940s. During her school years, she was the target of frequent bullying due to her pimply face and thick glasses, with a student named Olive Hornby being a particular tormentor. On 13 June 1943, a distraught Myrtle was hiding in a cubicle in the first-floor girls' bathroom, crying after Olive Hornby teased her about her glasses. While there, she heard a boy's voice speaking in a strange language, which was in fact Tom Riddle speaking Parseltongue to open the Chamber of Secrets. Annoyed by the male presence in the girls' lavatory, Myrtle opened the cubicle door to tell him to leave. Instead, she came face-to-face with the two large, yellow eyes of the Basilisk that had emerged from the sink. She died instantly, becoming the first and only student to be killed by the monster during its initial rampage. Tom Riddle used her murder to create his first Horcrux, his diary. After her death, Myrtle chose to return as a ghost and spent her early afterlife tormenting Olive Hornby, following her relentlessly until Hornby sought a restraining order from the Ministry of Magic. The Ministry intervened and ordered Myrtle to return to Hogwarts and remain at the scene of her death. She has haunted the girls' bathroom ever since, earning her “Moaning Myrtle” nickname by perpetually crying, wailing, and often flooding the bathroom by splashing water when in a temper. In Harry Potter and the Chamber of Secrets, Myrtle provides Harry Potter, Ron Weasley, and Hermione Granger with the crucial clue about her death—that she saw a pair of big yellow eyes in the tap—which helps them identify the creature as a Basilisk and locate the entrance to the Chamber of Secrets. In Harry Potter and the Goblet of Fire, she spies on Cedric Diggory in the Prefects' Bathroom and later advises Harry to open the Golden Egg underwater, helping him solve the clue for the second task. In Harry Potter and the Half-Blood Prince, Myrtle becomes a confidante to Draco Malfoy, who frequently seeks refuge in her bathroom to express his fear and despair over his mission from Lord Voldemort. She offers him comfort and is one of the few to witness his vulnerability. She later reports to Professor Snape that Harry used Sectumsempra on Malfoy.
As a ghost, Myrtle appears as a squat, translucent, pearly-white figure. She has lank, drab hair worn in pigtails, a glum face often marred by pimples, and wears the same thick, round glasses she wore in life. Myrtle's personality is defined by her perpetual misery. She is sulky, morose, and exceptionally sensitive, taking offence at the slightest perceived insult and prone to fits of wailing and sobbing. She is often found moping in the U-bend of her toilet. Despite her gloom, she can be quite helpful when she chooses, though she is also nosy, often spying on students using the bathrooms. She is lonely and craves attention, expressing a particular fondness for Harry Potter.
Little is known of Myrtle's magical abilities as a witch. As a ghost, she possesses the standard abilities of flight and intangibility, allowing her to float and pass through solid objects. Uniquely among the Hogwarts ghosts, Myrtle has a distinct ability to manipulate water, enabling her to splash, create waves, and cause the toilets in her bathroom to flood when she is upset.