Dr. Satan (real name S. Quentin Quale) was an insane surgeon in the town of Ruggsville. He conducted bizarre and inhumane experiments on the inmates of the local sanitarium, hoping to create a race of super-humans. According to Captain Spaulding, vigilante justice soon caught up with Dr. Satan and he was hung by a lynch mob.
Somehow he survived and fell in league with the Firefly family, living in underground catacombs somewhere on their land where his experiments continued with the help of his assistant, the Professor. The homicidal family would give Dr. Satan the victims they themselves did not want to torture anymore, including Jerry Goldsmith and Denise Willis (Satan would have also gotten Mary Knowles but she escaped and was killed by Baby).
Dr. Satan dissected Jerry, and Denise temporarily escaped from the Professor only to be capture by Captain Spaulding (who was in league with Satan and the Firefly clan) and Otis B. Driftwood. Dr. Satan operated on her when the Firefly family returned her to his secret lab. Although most of the Firefly family was either killed, captured or escaped when Sheriff Wydell of the Ruggsville County Sheriff's Department raided the family's house, the fates of Dr. Satan and the Professor remain unknown.
Grandpa Hugo was originally going to be revealed as Dr. Satan, but Rob Zombie changed the ending of House of 1000 Corpses to make Hugo and Satan separate characters.
Dr. Satan was originally going to appear in The Devil's Rejects but Rob Zombie cut his scenes from the film. These deleted scenes involved Satan being apprehended by the police after the siege of the Firefly family home and the evil old scientist confined to a bed in the hospital. When Sheriff Wydell's deputies came to question Satan, the surprisingly strong elderly doctor managed to kill his nurse in a failed escape attempt.