History
Originally, Hugo "Grandpa" Firefly was to be Dr. Satan. The legend of Dr. Satan was supposed to be just a story they used to lure their victims into their grasp. In the picture of Dr. Satan as an actual medical doctor in Captain Spaulding's Museum of Monsters and Madmen, the actor in the doctor's outfit was Dennis Fimple, who plays Hugo. When Rob Zombie decided to expand on the legend of Dr. Satan, this minimized Hugo's role in the film, and lead to him being portrayed as the most harmless of the Firefly family. He is presumably the father of Gloria "Moma" Firefly and the grandfather of Tiny, Baby, and Rufus. He does not engage in any acts of cruelty or sadism, but rather is just a harmless(yet dirty) old man. Dennis Fimple died shortly after filming of House of 1000 Corpses was complete. Rob Zombie omitted his role from The Devil's Rejects out of respect.
House of 1000 Corpses
Hugo "Grandpa" Firefly had a very crude, sexual humor and loved to tell dirty jokes. He was often seen yelling at the TV screen. When the family sits down to dinner with the four teenagers, he encourages them all to put their masks on, for dinner can't start until they do. After dinner, he opens a somewhat Halloween themed "talent show" with ghastly screams and a series of sexual jokes.
The Devil's Rejects
Grandpa "Hugo" Firefly did originally have a role in The Devil's Rejects. However Dennis Fimple died before filming of the movie had begun. Out of respect for Dennis, Rob Zombie omitted Hugo's role in the film.