Who he is
Rockstar sums Jason up in one line: he wants an easy life, but things keep getting harder. He grew up around grifters and crooks, did a stint in the Army to shake off a troubled adolescence, and came out of it in the Leonida Keys doing the one thing he knows - working for local drug runners.
Where he is when the story starts
In the Keys, working for local drug runners and living at a property owned by Brian Heder, a smuggler from the golden age of the trade. Brian lets Jason live rent-free at one of his properties in exchange for help with local shakedowns and the occasional visit for his wife Lori's sangria. His friend Cal Hampton is another of Brian's associates.
What he wants
Something new. Rockstar's bio ends on the thought that it might be time to try a different line of work - and the rest of the game is presumably what happens when he does.
Jason and Lucia
Rockstar puts it plainly: meeting Lucia could be the best or the worst thing to ever happen to him. Jason knows how he would like it to turn out. Lucia's bio says the same from the other side - a life with Jason could be her way out. Neither bio says it works.