Formerly a private security contractor in the Milky Way, Bain heard about the Andromeda Initiative from a friend. He was bored with his well-paying job and figured life in another galaxy would be a hell of an adventure.
He brought no family with him. Bain's mother died when he was a kid and he never met his father. All he knew is he was a big-time mercenary who gave him his name and nothing else.
If asked why he hates the kett, Bain simply points out they're ugly and offensive to his aesthetics. If asked about his role in the uprising, Bain explains he didn't like the way leadership played god and decided who to wake up. He lent his talents to the mutineers, and when the uprising failed, struck out on his own because he deemed most of the other exiles as unsavory assholes.
Bain's experience tracking kett tells him they are drawn to places with Remnant structures. He never discovered why despite questioning some, so he gave up and turned to killing them instead.
Mass Effect: Andromeda
Pathfinder Ryder can encounter Bain in a kett bunker strewn with corpses in the Presson Dunes of Eos, where he explains how to weaken the kett presence in preparation for a raid on the planet's main kett base.
If a female Ryder takes out Bain's recommended targets, he grows impressed and slightly flirts with her upon completion of the mission, starting with calling her "little duck", an appellation Ryder thinks is one of the worst ever.