Background
Val's mother died in childbirth. His father didn't want much to do with Val after that, he was a weak man. So Val has been on his own since he was a boy. While in school on Alderaan he learned he had an affinity for anything that moves fast. He also realized he could get most people to do the things he wanted simply by talking to him. This is how he survived. At 18 he had earned enough credits to purchase a ship and with it he discovered opportunity. It turns out people who were lets say less than loyal to the Empire would pay a premium to get off of Empire controlled planets, and Val's skillset seemed well suited to the task, so his life of crime began. In the beginning is was purely for the credits. To Val's mind one ruler was just as bad as the next. Since he didn't recognize their laws he reasoned it didn't much effect him. So Val continued to be Val. Then the Empire came for Alderaan. Be it destiny or just blind luck Val was offworld when the incident happened. While you might think his ties to Alderaan were limited and so he might not feel the weight of it as much, you would be wrong. Alderaan was the only anchor Val had to his mother. As his anger bubbled Val turned his skillset against the Empire. He began taking more dangerous jobs, opening his mouth less and reaching for his blaster more. Some jobs only involved his blaster. Val had fallen to a dark place. Val continued along this path until he met Kena.
Until this point he assumed he would never meet another survivor. So you might imagine the shock he felt when he was on what he thought was a rather boring and routine job to smuggle her offworld and she mentioned she was from his home. Over the course of the next months they grew close. He cracked wise and attempted to make light in very inappropriate situations. Much to Kena's chagrin. Mostly as an attempt to cope with all that has and will be lost. As they traveled and helped people in need, and killed troopers that needed killin, she confided in him that she was to rejoin the rebellion. And wanted him to come too. By the time they had tracked down the rebellion and landed (crashed) he had decided that becoming a rebel is just what he needed to do. Besides he had grown rather fond of Kena, plus she kept him out of trouble.....kinda.
Motivation
Connection No One: The Player Character is all alone in the galaxy. Whether he is just a loner or has lost everyone in a catastrophic Imperial attack, he has decided to join the Rebellion because he has nothing left to lose.
The destruction of Alderaan.