Human ghosts can arise when a human dies with unfulfilled regrets. An example of this is Selene, who accidentally released the Beelzebubs from their seal and was killed by them, but remained as a ghost in order to find someone capable of defeating the Beelzebubs. Human ghosts can also arise from humans whose bodies were used for necromancy, as in the case of the numerous ghosts who appeared as a result of Chrome's work. Human ghosts can imitate living humans well, as shown when Selene was not recognised as a ghost by either Luka or Alice. They also have the ability to transfer energy to others, something done by two ghosts to revitalize the San Ilia King.
Monstrous ghosts are ethereal beings composed of dark magic and souls. This composition makes them resistant to physical attacks, but it also makes them vulnerable to magic. There are various types of monstrous ghosts. Some, such as Page 17, possess books. These have the ability to draw a human man into an alternate universe within their book, in which they drain them for the rest of the man's life. Another type are mimics, such as the Mimic, which hide inside containers such as treasure chests. These exploit the greed of humans, preying on those who attempt to open them seeking treasure.
Ghosts can be summoned by mediums, to fight alongside them.