Ghosts

Ghosts (or spirits) are the disembodied spirits of the dead.

Background
Many traditions believe humans contain a soul that survives death. When that soul is trapped on the earthly plain (as opposed to being reincarnated or leaving earth for Heaven or Hell), it can sometimes be detected by the living.

Souls can become trapped on the earthly plain if they experience great violence, trauma, or confusion in their lifetime. They will often become attached to a particular house or location, perhaps where they died or a place that had a special meaning for them while they were living.

As such, ghosts are usually frightening and sad, for their fate is one of abandonment, confusion, and hopelessness. Some ghosts can be evil, desiring to harm or frighten the living, either because their souls are inherently evil, or because their suffering has caused them to become so. It is a misconception that ghosts cannot harm the living, for in the cases of several well-known haunted houses people have actually died from contact with the dead.

Behind the scenes
Ghosts are associated with "Gabriel Knight 4 (GK4)" according to Gabriel Knight 3.

Searching for 'spirits' will lead to the ghosts article as well. Spirits are referenced in Gabriel Knight 1.