Ghost

The Ghost is a Monster kept in the Facility. It is not listed on the Whiteboard, nor is it seen during the System Purge.

Physical Description and Attributes
The Ghost resembles a typical ghost. A faint, almost transparent figure, but with a grayish outline where some features can be pointed out, such as a nose, eyes and its basic silhouette.

It is briefly seen behind the Sugarplum Fairy's cube during the cube scene when the camera pans out to reveal all of the cube cells.

It is unknown how the Ghost kills or tortures its victims. It might bear some similar qualities or abilities to the Creeping Shadow or Wraiths, as they are ghostly figures as well. The name of the ghost, or rather the person it used to be, is unknown. It can be assumed that since this ghost is kept inside the Facility, it possesses some malevolent qualities and is harmful to humans, needing to be contained. Its possible the Ghost could be a type of evil spirit from a specific religion, such as a Jinn, Onryō, or some sort of poltergeist.

The item that summons the ghost from Cabin's Cellar is unknown, but its possible an Ouija board or some possession from the deceased person could summon it.

Inspiration
While there are thousands of popular horror movies where ghosts and evil spirits are the main antagonists, this ghost appears to not have been inspired by a specific movie ghost.