Background
Pash grew up on Corellia in a fairly middle-class lifestyle, but he was always attracted to danger, speed, and pushing the limits of authority. This led him to fall in with some bike gangs early on, and from there hooked up with the Grass Snakes, a loose collective of gangs whose activities ran from street racing to petty larceny to running jobs for the smugglers who would fly in and out of Corellia, and making connections between off-world goods and local fences. Pash discovered an aptitude for modifying vehicles and small spacecraft and gained some renown among the Snakes for his mechanical abilities, such that he was able to get more exposure to the gang's off-world interests, getting to know more smugglers and getting a chance to work on, and fly, the occasional starship.
In his later teens and early 20s Pash began to see more and more of the Empire's influence in Corellian society and came to feel that it needed to be checked. His approach to that was to make the Empire more directly a target of his gang activity - stealing vehicles from imperials on-planet, sabotaging their equipment, and befriending and aiding smugglers and others who's activities involved stealing from or otherwise hurting the empire. This brought him into contact with the Corellian pirate group, the Sand Panthers, but full piracy never sat well with him, so he preferred to assist smugglers, small time thieves and slicers, and his hometown gangs.
He built a small network of friends among the Snakes, including Octavia Kain who specialized in keeping track of what the empire was doing throughout the Corellian system, Zak Malo who would pick up work as a bodyguard for off-worlders who needed some muscle, Tel Rann who was a con-artist, fence and gambling fixer; and Lyl Dareen who he had a flirtatious rivalry with over who was the fastest driver/pilot.
As the empire's presence continued making the Snakes' lives more and more difficult, Pash began to consider making contact with some rebels and joining that cause. When Corana Loren, a smuggler he had aided in the past, appeared to be running goods for a corrupt mid-level imperial official in the sector, this was too much for him. He was hired to temporarily augment Corana's engines for a blockade run, and instead of confronting her about what looked like Imperial cargo, he instead sabotaged her engines, stole a starship belonging to an imperial bureaucrat, and headed off to make contact with a nearby rebel group that Octavia had given him information on.
Motivation
Pash is a bit of an adrenaline junkie and a show off, and tends to prefer risk to the safe route. He gets a rush from being in a tight spot, and won't pass up a chance to strike even when he's down.