Sidney Pen, also known as "The Fat Man" or "The Fat Fuck", is a crooked businessman and former associate of Don Alberto Clemente. Clemente bought a stake in Pen's Empire Bay Cab & Co. in the late 1920s and began laundering his bootlegging profits through the company. The two finally fell out when Pen opened a distillery in Sand Island in 1939 and refused to give Clemente a piece of the action, assuming the evidence he had against him would keep him safe. After his men killed two would-be hitmen sent by Clemente, Pen stepped up his personal security. After fending off a number of Clemente's hitmen, Vito Scaletta, Joe Barbaro, and Henry Tomasino are sent to eliminate him in 1945.
Vito, Joe, and Henry ambush Sidney's gang with an MG-42 outside the distillery, but he manages to flee inside. After shooting their way through his guards, they corner him at the top floor. He begs for his life, but as Henry is about to execute him, Sidney shoots him in the leg with a revolver. Joe and Vito then shot him through the chest at least 30 times, with a Thompson 1928 and a 1911 Semi, before escaping with Henry, as the factory is engulfed in flames.
- The Buzzsaw (killed)
- The Fat Man apparently has a family, as just before he is shot he says "I got a wife", though in the early trailers he said "I got a wife and kid".
- His real name is not revealed until the Mafia II Family Album is unlocked at the end of the game, with characters always using his aliases in-game.
- He is one of few characters to appear in the debut trailer for Mafia II in August 2007.
- The song The Fat Man by Fats Domino can be heard on Delta Radio shortly before his death.
- During the cutscene in which Joe and Vito kill him, his hat falls off. After the cutscene, he will have his hat back on.