Ghost

Ghosts are sentient undead beings featured throughout the JoJo's Bizarre Adventure series and the associated one-shot, Dead Man's Questions. In Part IV: Diamond is Unbreakable, they take on more significant roles as the series' subplot, even aiding the protagonists in unconventional ways.

Description
When a person, usually a protagonist, dies in the JoJo universe, often times their soul in the form of a incorporeal manifestation of themselves can be seen transcending into the Heaven. (Ex: Avdol, Iggy, Shigechi, Buccellati.) Individuals who's souls are incapable of passing through to the other side are forced to wander the earth as ghosts. Though each ghost's cause of death is different, most of them can be classified as Revenant: beings unable to enter Heaven and are essentially forced to roam the earth in a spiritual form, often times bound to a specific location or required to perform certain tasks.

As ghosts, normal people cannot see them and their bounds of human capability are removed, allowing them to perform various acts of supernatural abilities. The abilities vary for each individual, though most are able to fly. There is almost no difference in appearance when as a ghost (Exception being Kira), however some will retain their injuries from their death. (Ex: Arnold's neck is slit open and the injuries on Reimi's back are still present.) And though possibly fueled with revenge, ghosts will retain their personalities and are perfectly capable of reasonable thought.

Due to having spiritual manifestations themselves and a higher overall supernatural presence, Stand users are able to see ghosts and can even see and travel to ghostly locations such as Morioh's Ghost Alley. Users can also use their stand abilities on ghosts and/or attack them, (as seen when Rohan uses Heaven's Door on Reimi) though it is unknown whether or not this would damage the ghost.

Characteristics
Ghosts also need to follow certain rules in their existence. They can only enter a room if its empty or if someone inside the room has given the ghost permission to enter. They have the ability to manipulate objects, though they are not able feel them. Ghosts who previously had a Stand in their former, living life still can use them. It is also said that ghosts are unable to develop physically unlike living beings, and the memories stored in their spirit are only those of their former life (demonstrated when Rohan tried to read Reimi Sugimoto's ghost and only saw information from her life before dying). If a ghost touches someone by accident they lose their limbs, so most spirits just shrivel and hide in the shadows to avoid bumping into people. This does not appear to be the case with stand users or their stand abilities, however.

Objects
For some unknown reason, objects can become ghosts if destroyed as well. These can be felt and manipulated by normal ghosts, ranging from fire, food, books, guns, places and entire buildings. However, these objects aren't capable of spreading to other living objects, so it wouldn't be possible to kill someone using a ghost object. It is, however, possible to use them to kill cleansers.

Ghost Alley
A place in particular, known only as Ghost Alley, is the border between the world of the living and the afterlife, essentially serving as a purgatory. The area is just like a maze, but it can be escaped if given directions. There's only one rule to follow: never look back, or else hundreds of hands will drag the spirit. It is revealed that spirits dragged by the hands will become wandering vagabonds, without any memory of their past life and forced to find a purpose on Earth. Stands are destroyed when dragged by the hands, as shown with Killer Queen being torn to shreds and eventually not showing any traces of its existence later.

Cleansers
A Cleanser is a small creature that, as its name suggests, cleanses spirits. They are born from eggs touched by a spirit. When they touch a normal ghost, plants begin to grow from the ghost, trying to change them into something else similar to a plant by eating him from the inside. When they eat a ghost object, they turn those into bugs. They have the function to clean the world from spirits, so that it won't be overflowing with vagabond spirits of the dead. They can be killed either by cracking their eggs or with a ghost object, but their dead remains can still clean whatever it touches.

Under The Moon
The painting Under The Moon, featuring Nanase, was done by painter Nizaemon Yamamura with the blackest and purest ink in the world, taken from the pigment of an ancient tree's trunk. The pigment that Nizaemon extracted from the 2000 years old tree came from a substance taken from insects living in the very core of the millenium trunk. These infinitely black creatures, ressembling spiders, were hidden in the darkness since the dawn of time.

To use it on the painting, he cut down the tree, unaware that it was forbidden by decree to do so, and as a punishment Nizaemon was executed just after his wedding with Nanase. Having been forced to become a widow, Nanase became gravely ill and died soon after. According to legend, Yamamura had drawn the most evil and black painting in the world. That's because the hate in his heart for meeting such a fate was transfered and sealed to the painting by Nanase before her death, giving it life and a murderous intent. Because of this, Nanase was cursed to sacrifice people for 300 years and eventually seal the curse away using a descendant by cutting her link to said descendant.

When a living being draws near the painting, it perceives the heat or the breath of life and attacks the victim by using their sins, carved in their flesh, to create creatures similar to ghosts out of the victim's memories in order to mimic the appearances of someone close the victim may held some sin. This way, when someone tries to take a closer look at it, these monsters awaken and kill their victims in a similar manner the people they are copying died.

In 1989 it was acquired by the Louvre museum and kept in the Z-13 warehouse, which later was abandoned (probably because no one could get near the painting) with Under The Moon inside it. Rohan Kishibe, one of Nanase's descendants, finally managed to cut his link with her and seal the curse away by erasing all his memories with Heaven's Door. Under The Moon was eventually incinerated after analisys by scientists

Trivia

 * Aside from ghosts, the general definition of a revenant includes animated corpses, which would possibly explain the mysterious circumstances around Bruno Buccellati's death and would put him under the same category.
 * In Deadman's Questions, Araki explains that the the series' take on ghosts spawned from the idea that "In the world of life after death, if the spirit lives on, it would not be a place where everyone goes, but where there are "rules" similar to this world. The ghosts should go through just as much hardships as we do, if not more."