Dennis Tannen was a Galahad Arms agent and commanding officer operating in Carta.
Most of Dennis' life is unknown. He joined the Galahad Arms at one point, and convinced his brother, Mickey, to join. He became a commander in Dusktown at some point.
Following a shoot-out that led to Mickey's death, Dennis was called to the scene, vengeful over his brother's death. He led his men to a chase across Carta, chasing the Traveller's Tempest crew, who were marked as responsible.
Dennis' team lost their pursuees after they escaped the Cartan border. Dennis ordered his men to pursuit, despite their fear of the Sabarians, before leaving to inform Red Dragon. Before departing, Dennis saw Ken Javert departing into the desert wastes.
The following day, Dennis handed his report to Red Dragon, who got irritated at Dennis' and Mickey's failure to stop a small bunch of fugitives. Upon revealing that he spotted Ken Javert, Dennis was asked if he attempted to do anything regarding the commissioner, which Dennis denied. Disatisfied, Red Dragon cut Dennis' throat open, killing him.
Dennis is vengeful and caring towards his younger brother. He is fearful of Red Dragon.