"I know nothing of the secrets of death, Harry, for I chose my feeble imitation of life instead..."
-- Nearly Headless Nick (OP38)
"Wizards can leave an imprint of themselves upon the earth, to walk palely where their living selves once trod, but very few wizards choose that path."
-- Nearly Headless Nick (OP38)
Ghosts are an unusual type of magical creature. In some ways, they’re not creatures at all, but rather characters just like Harry, Ron, Dumbledore, and all the other living people. But they aren’t quite the same, of course; they are semi-transparent, non-corporeal beings. Only wizards can become ghosts, and then only if they choose that path before they die. They enter a state somewhere between living and dead. Nearly Headless Nick describes death as thus:
“Wizards can leave an imprint of themselves upon the earth, to walk palely where their living selves once trod … I was afraid of death. I chose to remain behind. I sometimes wonder whether I oughtn’t have … Well, that is neither here nor there … In fact, I am neither here nor there…” (OP38)
Ghosts can pass through solid objects. They do not eat, but in many other ways they seem to enjoy a full life. They interact with “live ‘uns,” as Sir Patrick said, referring to Harry, Ron, and Hermione. There are differences, however. It seems that their sensibilities are somewhat altered since their deaths, since their idea of music — an orchestra of thirty musical saws — is bizarre and dreadful to the ears of the living. Also, they do seem to be in some way tied to a place, some (Myrtle, for example) more than others. They don’t all get along exactly well, either. The Bloody Baron is avoided by most ghosts, and the Hogwarts ghosts don’t go into the Shrieking Shack because they say a “rough crowd” lives there (although this is likely a story they’ve invented to help support the official lore of the place).
While ghosts as a rule don’t affect the physical world — people walk right through them, for example — there are some exceptions. Myrtle can splash water out of the toilet she haunts when she wants to cause a ruckus to demonstrate how miserable she is. A ghost is very cold, and having one walk through you feels like a cold shower.
Ghosts can communicate with each other by ghostly letters. Nearly Headless Nick received one from Sir Patrick of the Headless Hunt. He must have used something of that sort to invite people to his deathday party, too (CS8).
The Ministry of Magic has some authority over ghostly behavior:
An exception was made for the ghosts, who asserted that it was insensitive to claw them as “beings” when they were so clearly “has-beens.” [Minister for Magic Grogan] Stump therefore created the three divisions of the Department for the Regulation and Control of Magical Creatures that exist today: the Beast Division, the Being Division, and the Spirit Division (FB).
When Myrtle was stalking Olive Hornby in retaliation for the way Olive tormented her in life, the Ministry was called in and Myrtle was forced to return to Hogwarts and haunt the place of her death (GF25).
Nick indicated that wizards study the matter in the Department of Mysteries, which may be a reference to the veiled archway Harry saw there (OP35).
There are at least twenty ghosts at Hogwarts (PS7). When ghost-related decisions must be made, they hold a “ghost’s council” (GF12). The ghosts attend Hogwarts feasts and they stand out among the crowd because they are pearly white and glow (PA9).
- Nearly Headless Nick (Gryffindor ghost)
- The Bloody Baron (Slytherin ghost)
- The Fat Friar (Hufflepuff ghost)
- Moaning Myrtle
- Peeves the poltergeist (Peeves is not a technically ghost, he is a poltergeist, but the Hogwarts ghosts do grudgingly take some responsibility for him)
- The Grey Lady (Ravenclaw ghost)
- Professor Binns
Ghostly Guests at the Deathday Party
On the 500th anniversary of Nearly Headless Nick’s deathday a party was held in the Hogwarts dungeons (CS8). Harry, Ron and Hermione were special guests along with hundreds of ghosts, including all the Hogwarts ghosts and …
This ghost in chains could be Jacob Marley, the ghost of Ebenezer Scrooge's dead partner, from Charles Dickens' A Christmas Carol. Thanks to Mike Ball for this suggestion.
The ghost knight with the arrow through his head, it is possible that this is the ghost of cHarold II (1022? - 1066). He was the last Saxon king of England. He was defeated at the Battle of Hastings in 1066 by William the Conqueror. Harold was killed when an arrow pierced his eye. Thanks to Mike Ball again for this excellent deduction.
We know that there are animal ghosts because the Headless Hunt rides ghost horses. But we are told in OP38 that only witches and wizards can become ghosts; some sort of preparation or planning seems to be required. So, most likely, animals can become ghosts if a human performs the necessary magic on them. -BB
From the Web
"Ghost" on the Harry Potter Wiki
"Ghosts" on Pottermore
"Hogwarts Ghosts" on Pottermore
"Pottermore's Guide to Ghosts" on Pottermore