Mobster ghosts aren't exactly rare, but Knuckles presents a level of professionalism that's a cut above the rest. He doesn't put a severed horse's head in an enemy's bed - he uses his own.
Knuckles is a spectre. In life, he was one of Don Bartholemew's gangsters. One day, while playing cards, he discovered that the croupier - Eddie Venus - had rigged all of his games to insure that he never lost any of the don's money. Not wanting his secret to get out, Eddie shot Knuckles dead. His spirit is now bound to the gambling table he was killed over, where he seeks revenge against the cheating croupier.
Knuckles can be unlocked in Deadfellas. To free him, he wants Eddie Venus to lose all his money. This is, unfortunately, next to impossible since Eddie has rigged his roulette table so that no one can win unless they're very lucky. To this end, have a ghosts use powers like Charm, Luckstorm, and Jinx to alter everyone's luck. If Eddie is made unlucky or his player is made lucky, then his roulette table will start paying out, allowing the player to win despite all the cheating, causing Eddie to get more and more afraid. After a player has won three times, Eddie will have lost all his money.