Ghost

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

Summary
When a person dies their soul or spirit (魂) passes on to the afterlife. In the case of a fundamentally virtuous person such as Avdol, Iggy or Bucciarati 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, many of them can be classified as Jibakurei (地縛霊) or, beings typically bound to a specific location on Earth due to 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 Yoshikage 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. While 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, they can still make themselves heard by normal people. Stand users are able to see ghosts automatically and, as Reimi theorizes, their power allows them to see and travel to ghostly locations such as Morioh's Ghost Girl's Alley. Due to their high level of spiritual excellence, monks can also see ghosts with no problems. Stand 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 them.

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 can freely toggle intangibility. By default, it seems that they 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.

Ghost Girl's Alley
In the Japanese town of Morioh, a particular place nicknamed the Ghost Girl's Alley is said to be the border between the world of the living and the afterlife, essentially serving as a purgatory. Not only the alley is a closed space, but malicious spirits lurk in it. Any soul passing through here should never look back or else countless disembodied hands will drag the spirit into the unknown.

In the case of Yoshikage Kira 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. 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.

Burning Down the House
Burning Down the House is a Stand that allows its user, Emporio Alnino, to interact with the ghost of a room as well as various ghost objects, such as a computer, a piano, a gun, and food. Burning Down the House's room and objects share the same or similar rules to other ghost objects and locations, such as them not being able to affect living objects. Emporio is also able to bring in other people into the room as well and he is able to enter and hide inside Burning Down the House through cracks in the walls of Green Dolphin Street Prison.

Vengeful Spirits
Vengeful spirits are depicted as supernatural entities born from grudges of a previous life, which break some of the established conventions of common ghosts; for example, these spirits are capable of interacting with non-Stand users and can be seen by them. When The Confessor unintentionally causes the death of a vagrant, the vagrant reappears as a spirit cursing the confessor for his cruelty. The vagrant's spirit demonstrates super-human capabilities, being capable of knocking away a fully grown man and slicing the head of the Confessor's Servant clean off. They are also able to possess the bodies of people. To pass on to the next life, these spirits need to satiate their grudge.

Spirit Cleansers
Spirit Cleansers (魂の掃除屋) are small creatures that, as its name suggests, cleanse 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 created 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-year-old tree came from a substance taken from insects living in the very core of the trunk. These infinitely black creatures, resembling spiders, were hidden in the darkness since the dawn of time.

According to legend, Nizaemon had drawn the most evil and black painting in the world. The hate in his heart for being executed was transferred and sealed to the painting, giving it life and a murderous intent. As a result, 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.

The painting can sense when someone approaches it, and attacks them by summoning creatures similar to ghosts out of the victim's memories such as their ancestors or others who died in horrific ways. If their victim touches a ghost created by the painting, the victim dies by experiencing the same death as the ghost.

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Trivia

 * In Deadman's Questions, Araki explains that 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."