Sir Nicholas de Mimsy-Porpington, or Nearly Headless Nick, was the Gryffindor house ghost during Harry's time at Hogwarts. He was a wizard in the royal court before he died. His neck is only partially attached to his head by a flap of skin.
Nearly Headless Nick is the Gryffindor house ghost. Though he prefers to be called by his real name - Sir Nicholas de Mimsy Porpington - everyone refers to him by his nickname, which he received due to not being beheaded properly. His ghostly head swings to the side as though on a hinge.
Nearly Headless Nick introduces himself to the new Gryffindor students at the feast. He looks mournfully at the food, before telling Harry that he hasn't eaten for nearly five-hundred years. He slightly resents his nickname, and looks irritated as he is asked to explain why he is only 'nearly headless'. He is nevertheless pleased by the stunned looks of the first year students.
Nearly Headless Nick is moping at having been turned down for the Headless Hunt when he meets Harry in the corridor after Quidditch practice. After helping Harry out of a tight spot with Filch, he invites him to his Deathday Party on Hallowe'en and Harry accepts. The party is held in a dungeon, and is full of pearly-white ghosts who have travelled from all over the country to attend.
Along with the rest of the Hogwarts ghosts, Nearly Headless Nick provides entertainment at the end of the Hallowe'en feast. The ghosts pop out of the walls, before performing some formation gliding. Nearly Headless Nick then re-enacts his own botched beheading, to great success.
Nearly Headless Nick attends the Sorting Ceremony dressed in his usual doublet, with a particularly large ruff, which serves the dual purpose of looking very festive, and not allowing his head to wobble too much on his neck. He greets the students eagerly as they take their seats at the Gryffindor table. He tells Harry and Ron that he hopes the new first-year Gryffindor students are up to scratch, as they want to continue their winning streak this year. Nick looks mournful as the feast begins and Harry, Ron and Hermione begin filling their plates, as he cannot eat. He tells the trio that they are lucky there is a feast at all that night, as Peeves caused mayhem in the kitchens after the ghosts' council refused to let him attend the feast. Nick is surprised by Hermione's reaction to his admission that there are house-elves working at Hogwarts. He is amused when she asks him whether the house-elves get paid, or are allowed holidays, laughing so hard that his ruff slips and his head flops off, dangling on the inch or so of skin that keeps it attached to his neck.