Ghost

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

Description
When a person dies in the JoJo universe, their soul passes on to the afterlife. In the case of a fundamentally virtuous person, such as Avdol, Iggy, Shigechi or Bucciarati, often times, their soul, in the form of an incorporeal manifestation of themselves, can be seen transcending into the Heaven.

Individuals whose 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 Revenants: beings unable to enter Heaven and are essentially forced to roam the Earth in a spiritual form, still bound to Earth because of their determination to accomplish a particular goal or not having found peace.

There is almost no difference in appearance between someone and their ghost, as the souls are reflections of the person (one exception being Kira, who has extraordinary circumstances). Moreover, a ghost will generally keep the injuries from their death, but a recollection of how they died is required before the damage on their body appears. 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's ghost and only saw information from her former life). And though possibly fueled with revenge, Ghosts will retain their personalities and are perfectly capable of reasonable thought.

Ghosts are invisible to ordinary people but not to animals. However, Ghosts can still make themselves heard by normal people. Due to having spiritual manifestations themselves and a higher overall supernatural presence, Stand Users are able to see ghosts automatically and can even see and travel to ghostly locations such as Morioh's Ghost Girl's 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 inflict pain onto the ghost.

As seen in Dead Man's Questions, Ghosts are rather common and can be found anywhere. Most tend to remain inactive and simply find a place where they are not likely to encounter the living.

Abilities and restrictions
Ghosts follow certain rules in their existence for unexplained reasons. For instance, they can only enter a room if it's empty or if someone inside has given the Ghost permission. However, the nature of the permission is loose: even accepting to open a mail slot counts as inviting a Ghost.

Ghosts have freely togglable intangibility. By default, it seems that Ghosts are intangible and thus can freely move through objects. However, they still have the ability to manipulate objects, though they are not able to feel them. Should a Ghost touch someone unwillingly, they lose their limbs, so most spirits just shrivel and hide in the shadows and out-of-the-way places to avoid bumping into people.

Ghosts who previously had a Stand in life can still use them, provided they do not become vagabond spirits, as the Stand is broken apart and destroyed upon the transition.

Inanimate Objects
For some unknown reason, objects can become Ghosts if destroyed as well. Worded differently, the objects become "made of Ghost" as if material. 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 other spirits such as cleansers.

Ghostly locations such as Ghost Girl's Alley or Emporio's room are hidden from living humans as they somehow fit into narrow spaces such as the space between two adjacent buildings.

Burning Down the House
Burning Down the House is a Stand from Stone Ocean that allows its user, Emporio Alniño, to interact with the ghost of a room as well as various ghost objects, such as a computer, a piano, a gun, etc.. Burning Down the House's room and objects share the same or similar rules as the ghost house and objects from Dead Man's Questions, such as them not being able to affect living objects. Emporio is also able to bring in other people into the room as well. Emporio is able to enter and hide inside Burning Down the House through cracks in the walls of Green Dolphin Street Jail, much like how Kira found and entered the ghost house in Dead Man's Questions.

Ghost Girl's Alley
A place in particular, located in the Japanese town of Morioh and known only as Ghost Girl's 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 countless hands will drag the spirit into the unknown. In Dead Man's Questions, it is revealed that spirits dragged by the hands, at least in Kira's case, 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. It seems that the Ghost Alley itself, the hands of the alley, or some different entity altogether can talk as Koichi hearing something whisper that he can turn around when he was near the end of the maze.

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, resembling 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 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 transferred 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 supposedly incinerated after analysis by scientists, though Rohan explains that he can't know for certain if this is true.

Trivia

 * Aside from ghosts, the general definition of a revenant includes animated corpses, which also describes the mysterious circumstances around Bruno Bucciarati's death and revival with assistance from Giorno Giovanna, 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."