Jason Duval
Co-protagonist — male lead, partner to Lucia Caminos
Jason wants an easy life, but things just keep getting harder. He grew up around grifters and crooks, served a stint in the Army to escape his troubled teenage years, and ended up working for drug runners in the Leonida Keys. Meeting Lucia could be the best or worst thing to ever happen to him — Rockstar has said their bond sits at the emotional center of the game.
Jason is written as the tired one. That is unusual for a GTA lead, and it is the single most useful thing to understand about him — the series normally opens with someone who wants in, and this time it opens with someone who wants out.
A man who keeps choosing the easier wrong thing
The biography Rockstar has given him is a chain of exits that turned into traps. He grew up around grifters and crooks, joined the Army to get away from a bad adolescence, and came out of it working for drug runners in the Leonida Keys. Every step was a way to escape the previous one, and every step landed him somewhere worse.
That is a specific kind of character, and it is not Michael De Santa from GTA 5, who wanted his old life back. Jason does not appear to be nostalgic for anything. He appears to want the current thing to stop.
What the Combat Focus ability tells you
Jason's special ability slows time in gunfights and highlights weak points. Mechanically that is a marksman tool — it rewards deliberate, accurate shooting rather than volume of fire.
Compare that to what has been said about Lucia and a picture forms of two leads who are meant to be played differently rather than being reskins of each other. In GTA 5, the three protagonists' abilities did meaningfully change how each one felt to control, and the same design intent looks present here.
What is confirmed and what is not
Confirmed: he is a playable co-protagonist, the background above, the relationship with Lucia Caminos, and the Combat Focus ability. Rockstar has described their bond as the emotional centre of the game.
Not confirmed: his voice actor, how the story ends for him, whether the two leads can be separated for long stretches, and every theory you have seen about who betrays whom. There is a lot of confident content online about Jason's arc. None of it is sourced, because there is nothing to source it to yet.