Nearly Headless Nick is one of the House Ghosts of Hogwarts, the wizarding school of the Harry Potter novels. He is the Resident Ghost of Gryffindor Tower.
*May contain spoilers*
Nearly Headless Nick introduced himself to Harry Potter and the other first years at the start-of-term banquet. He prefers to be addressed by his proper name, Sir Nicholas de Mimsy-Porpington. He died by beheading, however a strip of flesh remained to hold the head to the neck.
Nearly Headless Nick died on October 31st, 1492, as celebrated at the Deathday party that Harry Potter, Ron Weasley and Hermione Granger attend in Harry Potter and the Chamber of Secrets. One of his greatest desires is to join the Headless Hunt, an association of ghosts. However, he is being denied admission because his head does remain attached to his body, albeit only slightly. Because of this, Nick holds a grudge against Sir Patrick, the leader of the Headless Hunt.
Nick will work to assist any new Gryffindor that comes to Hogwarts, and finds Peeves the poltergeist to be a large nuisance.