A ghost is a type of supernatural creature, created when someone with excess soul dies without attempting werewolf or vampire metamorphosis. They are relatively rare, and more likely to be women (because of the survivability problems with female metamorphosis).
Their state of mind relies on the physical state of their body, to which they are tethered and cannot move far from. As the ghost's brain decomposes, the ghost slowly goes insane, entering a "poltergeist" phase before eventually disanimating completely. If a ghost's body is properly preserved, their "lifespan" can be years after their original death, and they can retain their sense of self, mind, and reasoning. Their form will remain stable in this condition, as well.
Ghosts cannot be preserved indefinitely, however. If the ghost's body is immersed entirely in formaldehyde, the tether between the ghost and its body cannot stay intact. Madame Lefoux solves this problem and drastically slows her aunt's decomposition by injecting air into a vat of formaldehyde in enough quantity to keep the tether while submerging the body. Unfortunately, the body can still decompose in this state, though very slowly, and Formerly Lefoux eventually loses her sanity.
Once the inevitable rot of their bodies begins, however, ghosts slowly begin to go mad. Their ghostly appearance will begin to lose form and separate, leaving parts of them floating around independently. In these instances, an exorcist may need to be called to put the ghost to rest. Only a preternatural can exorcise a ghost. They do so by coming into physical contact with the ghost's dead body.