The Unnamed Accomplice's backstory is unknown. At some point prior to 2001, he became affiliated with the Colombian Cartel. He became a gang member within the Cartel.
Events of GTA III
In October of 2001, the accomplice assisted Claude and Catalina in robbing a bank. As Miguel stopped his Banshee in front of the bank, the accomplice and Claude were both shot, with only the latter surviving. Catalina and Miguel escaped in the Banshee, and Claude was arrested, thus starting the events of Grand Theft Auto III.
The Accomplice uses the blue generic Cartel Pedestrians character model, and is depicted as a low-life gangster. He wears a blue shirt embroidered with palm trees, jeans, and a dark gray hat with a blue strap.