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.) Though each ghost is different in terms of cause of death, 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 from 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 during the moment of 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.

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."