Summary
In theHarry Potterlore, a ghost is described as a transparent, three-dimensional imprint of a deceased witch or wizard who exists in the mortal realm due to some “unfinished business”. Even though ghosts have some semblance of a physical form in the land of the living, they exist “neither here nor there”. In fact, only a few in the wizarding world opt for this “feeble imitation of life” after death.
While there are many ghosts atHogwarts School of Witchcraft and Wizardry— like Moaning Myrtle and Professor Binns, to name a few — the house ghosts are most notable in theHarry Potterseries: Nearly Headless Nick (Gryffindor), the Fat Friar (Hufflepuff), the Grey Lady (Ravenclaw) and the Bloody Baron (Slytherin).

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Nearly Headless Nick
Sir Nicholas de Mimsy-Porpingtonis the only house ghost who did not live during the time ofthe four Hogwarts’ founders. He was born into a noble family during the fifteenth century, and attended Hogwarts as a child, where he was sorted into Gryffindor. Although he was a “less accomplished wizard than he believed”, he went on to be knighted when he was a courtier at King Henry VII’s royal court. While strolling in the park one evening with Lady Grieve, a lady-in-waiting, he foolishly attempted to straighten her crooked teeth with a charm. The spell backfired, causing her to sprout tusks instead. Nicholas was immediately stripped of his wand and sentenced to death on 31st October, 1492. However, his executioner’s ax was blunt, and it took forty-five blows to kill him. There was still “half an inch of skin and sinew” holding his neck on — earning him the name ‘Nearly Headless Nick’. In an early draft ofHarry Potter and the Chamber of Secrets, he authored “The Ballad of Nearly Headless Nick”, which told the tale of his botched beheading.
Regardless, he is a jolly and friendly ghost, and even reenacts his death for entertainment during one Halloween feast. He still carries himself in a prim manner and prefers to be addressed as ‘Sir Nicholas’, as he is proud of his “noble blood”. He does feel inadequate about not being a truly headless ghost though, especially after his application to join the Headless Hunt is rejected. He is also one of the victims who get petrified by the Basilisk, but it is unclear how the Mandrake Restorative Draught is administered to the ghost for revival.

Nick also proves to be helpful to Harry on a number of occasions, declaring that he “would rather die than betray his trust”.When Sirius Black dies, Nick explains to Harry why his godfather would have “gone on” instead of returning as a ghost like him, who only “chose to remain behind” as he was “afraid of death”.
The Fat Friar
The Fat Friar of Hufflepuff is the only house ghost whose backstory has not been discussed in the books. J. K. Rowling revealed later on that he was also a student at Hogwarts,taught by Helga Hufflepuff herself. He joined the clergy afterward and devoted his life to religion. He had a kind demeanor, but this eventually proved to be his undoing. The senior churchmen had become suspicious of his ability to cure peasants with pox by merely poking them with a stick. This — along with his penchant for pulling rabbits out of the communion cup — eventually led to his execution. It is said that he always resented the fact that he never became a cardinal.
The Fat Friar is quite a merry and “genial” ghost. His forgiving nature is illustrated when a ghost’s council is held to decide whetherPeeves, the troublesome poltergeist at Hogwarts, should be allowed to attend the start-of-term feast, and he is the only one who believes in giving him a chance.

The Grey Lady
The Grey Lady (originally named the ‘Whispering Lady’ in the early drafts) was the daughter of Rowena Ravenclaw, and her actual name was Helena. She attended Hogwarts, where she was, of course, sorted into Ravenclaw. Even though she was “long-haired and beautiful”, she usually wore a “haughty and proud” expression. Helena was envious ofher mother’s cleverness and influencein the wizarding world, and ran away with her diadem while she was still at school. Rowena was ashamed of her daughter’s betrayal and kept it a secret. When she got fatally ill, she sent a man (later known as the Bloody Baron) to find her as she wished to see her daughter one last time. The Baron had long loved Helena, even after she had rejected his advances. When he tracked her down to a forest in Albania, she refused to go back with him — upon which he stabbed her in a fit of rage. Her ghost returned to Hogwarts, with “a single dark wound in her white chest”.
The Grey Lady seems mostly timid and quiet, even though the Ravenclaws claim that she maintains friendly relations with them, and is “particularly useful” if someone is lost or has misplaced something. She also proves to be instrumental inHarry Potter and the Deathly Hallowswhen she discloses to Harry where the diadem (one of Voldemort’s Horcruxes) is. In fact, it was her who chose to tell an “understanding” and “sympathetic” Tom Riddle years ago that she had hidden the diadem in a hollowed tree of the same forest in Albania — a decision that she came to regret later on.

The Bloody Baron
Little is known about the Bloody Baron’s early life. Born no earlier than 982, he was sorted into Slytherin at Hogwarts, where he was taught by the founder of his house. He had always been a “hot-tempered man”, and was prone to sudden bursts of anger when contradicted. Similar to the Grey Lady, he also died in the eleventh century when he committed suicide with the same knife he had used to kill her. Overcome with grief and regret, his ghost wears heavy chains “as an act of penitence” while drifting around at Hogwarts. No one else seems to know why his robes are covered in silver bloodstains, nor does anyone dare ask him. Even Harry witnesses at Nearly Headless Nick’s 500th Deathday Party that the Bloody Baron is “being given a wide berth by the other ghosts”. His favorite pastime is just “groaning and clanking up on the Astronomy Tower”. Due to his frightening and violent nature, he is the only one at Hogwarts who can actually control Peeves.