"A very good Boo to you, my friend."
A ghost is a disembodied soul of a dead creature, either a former adventurer once controlled by a poorly skilled player, or a monster unfortunate enough to bump into an adventurer skilled enough. That basically means all ghosts are derived undead, creatures without given attributes who inherit all their stats from their former living selves.
Of course they are upset, doomed to afterlife in the Hell and longing for companions - and you should be the one to accompany them on their way. Ghosts no longer have physical bodies, they are but barely visible floating silhouettes of ectoplasm, passing through walls with eerie soundlessness. Ghosts naturally cast a dull white light around them, not normally noticed unless you lack a light source.
Below you can find statistics of an example ghost.
|Random Stats||-||Permanent States|
|Size||100||Infravision, ESP, Gas Immunity, Levitation, Ethereal|
When one of your characters dies, he does not disappear forever. You may meet him again in a ghost form, disturbed and angry with the former player (you). In effect, the level you die on is saved and can be reloaded into another later game, such levels being known as bone files.
Each time you enter any level the first time in the given playthrough and you have died here in any previous game, there is a small chance that instead on generating a new level, a bone file will be used. That way you can recover items you had on your old adventurer, but all other features on that level will be saved as well - the monster that had killed you, all physical alterations to the dungeon made by you etc.