|The following is based on Fallout: Brotherhood of Steel and has not been confirmed by canon sources.|
Hieronymous is a ghoul merchant that inhabits the first area of Los the player comes to. He is not very civil to the player when you meet him but he offers the abilty to fight in the Gladiator Pit for a few caps, and sells weapons such as the combat shotgun and combat armor.
He has a twin brother Salieri, whose ghoul form is nearly identical to him. Hieronymous dies when Salieri sends an explosive package to him to eliminate competition.
Hieronymous appears only in Fallout: Brotherhood of Steel.