In ghost movies like The Devil's Backbone, the ghost is an earthbound spirit who is prevented from moving into the true sense of death. It is a common theme in ghost narratives for the transition to true death to occur only when the conditions of the ghost’s torment and unrest are satisfied, allowing the dead to finally rest in peace.
This kind of ghost has been referred to as a humanist ghost. The humanist ghost is a ghost that is still bound in the human psyche, with human issues and human concerns. The Others (2001), The Sixth Sense (1999), Ghost (1990), are other examples of humanist ghost narratives.
In her study of ghostly cinema, Aviva Briefel labels films like The Others and The Sixth Sense as “spectral incognizance”, a sub-genre of ghost films where the ghost needs to realise they are dead in order to pass onto the netherworlds. These ghosts need the help of humans and the earthy realm to realise their own deaths and aid their spectral transition. While The Devil's Backbone does not belong to the spectral incognizance sub-genre, Santi does require the connection and interaction with the human world to allow him to rest in peace.
As Santi leads Carlos to the truth of his death, the darkest elements of the human condition are brought to light. Jacinto’s greed and treacherous violence, Carmen’s shame at her own lust for him, Jaime’s fear and subsequent guilt at knowing the truth of Santi’s death. Del Toro states that The Devil's Backbone is a lesson “that you shouldn’t be afraid of a ghost – you should be afraid of the living.” As Carlos helps Santi reveal the truth of his death, Santi helps the living world confront their own haunted lives and selves. This is a truth common to a great many ghost stories. In Poltergeist (1982), the spirits were restless and angry because the thoughtless greed of the property developers had disturbed their graves. In Ghost, Sam remains after death in order to warn and save Molly from the people who killed him. Though not specifically a ghost, Eric Draven in The Crow (1994) returns from death to avenge his and his fiancée’s murder. In Shakespeare’s Hamlet, the ghost of Hamlet’s father tells him the truth of his death and implores his son reveal Claudius’ guilt.