Barrett serves as Murphy’s bodyguard and assistant as the ghoul scientist attempts to synthesize and sell Ultrajet, a potent chem.[1] He is not very vocal, and not even Murphy knows about his past, which Barrett is more than content with.[Non-game 1]
↑The Lone Wanderer: "What's the deal with Barrett?" Murphy: "He's my assistant and my backup. In case things get rough. I wouldn't mess with him if I were you." (Murphy’s dialogue)
Non-game
↑Fallout 3 Official Game Guide Game of the Year Edition p.61: "Barrett: Murphy's bodyguard doesn't "do" chitchat. Not even Murphy knows about his past, and Barrett is quite happy to have it stay that way."