Regi spawns after a certain level is reached in a bombed out building west of the Corvega assembly plant with a small army of radroaches. He carries either a minigun, Gatling laser or another legendary item based on player character's level.
His roach army, and the fact that his name roughly translates to King Roach (Regi from Regis, while Blattaria is the Latin name for cockroach) while he is carrying a minigun all imply he is a reference to The Roach King from Fallout 3. There is even a toy rocket reminiscent of the Roach King's throne on a shelf near his spawn point. There's also an ushanka hat like the Roach King wore. To support this, within the game's code his Editor ID is "POIRoachKing".