Hieronymous is a ghoul living in Los in 2208.
He is a merchant that inhabits the first area of Los the player comes to. He is not very civil to the Initiate when they first meet, but he offers the ability 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.