Official Rockstar Games clip, slowed down
Protagonist
Jason Duval
He has spent his whole life looking for the easy way out, and never finds it.
Official screenshots by Rockstar Games
Who they are
Jason grew up among grifters and small-time crooks. A spell in the Army was meant to put a difficult adolescence behind him; it left him in the Leonida Keys doing the one thing he is good at, working for the people who move drugs through the area.
He lives rent-free in one of Brian Heder's properties in exchange for helping with local shakedowns and turning up for Lori's sangria once in a while. It is a comfortable routine going nowhere, and he knows it.
His circle is small and it shows: Brian, who gives him a roof and a job, and Cal Hampton, who would rather be at home with the Coast Guard frequencies than anywhere near a plan. Neither of them is getting him out of this.
Then Lucia turns up. Rockstar says it plainly: meeting her could be the best thing that ever happened to him or the worst, and Jason himself cannot tell which.
What is known
He is one of the two playable characters: you switch between him and Lucia anywhere in the open world, except during certain stretches of the story.
You can call or text him to come to where you are. In a car they swap seats: one drives, the other shoots.
He lives rent-free in one of Brian Heder's properties in exchange for helping with local shakedowns.
“If anything happens, I'm right behind you.”
Official artworkOfficial artwork by Rockstar Games — “Jason and Lucia 01” Rockstar has only published artwork of Jason and Lucia together, so this portrait is a crop of one of those official pieces.
In the game
Jason beside a bike in the Keys, in broad daylight. 
With Brian Heder and Cal Hampton in a bar. 
Behind a fence, with police lights behind him. 
Indoors, with a weapon and spent casings on the table.
Official screenshots by Rockstar Games