|The following is based on Fallout: Brotherhood of Steel and has not been confirmed by canon sources.|
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.
- Deliver a package - Salieri sends the player out to deliver an explosive package to his brother Hieronymous, in order to eliminate competition.
- Gladiator Pit - Defeat enemies in the pit, 21 rounds maximum, 50,000 caps max reward.
Hieronymous appears only in Fallout: Brotherhood of Steel.