: The narrator must decide whether to exorcise the demon—potentially killing Prosper and robbing the world of his genius—or allow the possession to continue for the sake of humanity's cultural advancement. Legitimate Access

: The exorcist encounters Prosper of Schanz , a brilliant philosopher and polymath modeled after Leonardo da Vinci. He realizes that Prosper is possessed by a demon whose influence is responsible for Prosper's incredible artistic and scientific achievements.

: An unnamed, morally ambiguous exorcist who can see and communicate with demons. He is sanctioned by the church but is widely disliked because his methods of extraction often result in the death or permanent injury of the human host.