A master trader and barterer, Jack is a jovial sort, and not just because he's set up shop next to the raider strip club. He mends the raider's equipment, which keeps him busy and unable to continue modifications on his pride and joy: The Terrible Shotgun that he whips out for a little fun.
Despite being a raider, Smiling Jack is not aggressive to the player (unless provoked). This is because as a trader, he doesn't care who buys or sells from his store, so long as the caps are there. He holds no loyalty to the raiders he lives with, as he does not turn aggressive even if the player kills them in his sight. Still, he urges the player not to let them see him trading with an outsider, likely fearing that the raiders would turn on him if they were caught.
Smiling Jack is an obvious reference to Smiling Jack from Vampire: The Masquerade - Bloodlines, a game developed by former Fallout developers Tim Cain and Jason Anderson. Besides sharing the same name and both having a jovial disposition, Smiling Jack warns the player character in Vampire to 'watch out for shotguns' as they are more volatile to vampires than most guns.