Myrtle Warren

Moaning Myrtle is a ghost that haunts the second floor girl's bathroom (and occasionaly other bathroom facilities) at Hogwarts School for Witchcraft and Wizardry.

Myrtle died when the Chamber of Secrets was opened for the first time. The monster that killed her was a basilisk.

She helps Harry Potter, Ron Weasley and Hermione Granger to find out about the monster inside the Chamber of Secrets.

Myrtle also helps Harry with his second task in the Triwizard Tournament. Though Cedric Diggory had told Harry to take a bath while thinking about it, and provided him with the password to the prefect's bathroom; it was Myrtle who told Harry how to solve the clue, by opening it underwater.

She also made an appearance in Harry Potter and the Half-Blood Prince as somewhat of a confidante to Draco Malfoy.

Beyond Book
Shirley Henderson played Moaning Myrtle in the movie version of Chamber of Secrets and in Goblet of Fire.