The Gryffindor ghost, named because his beheading wasn't entirely successful, and his head is still attached to his body by a small bit of flesh. Harry seeks Nick out after Sirius dies to ask him if it's possible for Sirius to come back as a ghost. Nick sadly explains that becoming a ghost is only something that people who fear death to an extreme degree choose, and most people pass on.
Nearly Headless Nick Character Timeline in Harry Potter and the Order of the Phoenix
The timeline below shows where the character Nearly Headless Nick appears in Harry Potter and the Order of the Phoenix. The colored dots and icons indicate which themes are associated with that appearance.
When the sorting is over, Dumbledore announces the start of the feast. Hermione and Nick discuss the Sorting Hat's warning, and Nick suggests that since the hat lives in Dumbledore's... (full context)
...and cover themselves in the Invisibility Cloak to walk to Hagrid's. They pass Nearly Headless Nick humming "Weasley is our King" and shuffle through the snow. Hagrid lets them in and... (full context)
...to talk to Sirius. He races down to the entrance hall and finds Nearly Headless Nick. Nick seems to have been expecting Harry and acts uncomfortable. Nick says that students often... (full context)