Simon Fletcher is an agent of A1Zero.
That interestingly named small organization is a group working "under the umbrella" of a private security company started by an old frenemy of Fletcher, Sean Smith. Smith had been in charge of the military force Fletcher had been with in Afghanistan handing mission with and for the CIA as well as the Pentagon. When the rapid exiting of that country by American forces took place, Fletcher had "stayed behind to back proxy armies and assist with Afghan militia allies, opposed to Taliban rule" but he soon grew too tired of taking orders from the suits in D.C.
Fletcher was in Thailand, getting into trouble with some powerful people for reasons Fletcher did not understand, when Smith tracked him down and offered him a position in A1Zero. As Smith puts it, "The CIA awarded my services with classified contracts, providing a list of targets and you're the man I'm choosing to do the dirty work up close and personal". Smith's reasoning for picking Fletcher was summed nicely by "You're a patriot, Fletch. God doesn't make men like you too often. He's given you a talent, and now I'm giving you a purpose". That talent, by the way, is killing bad guys and as Smith rightly puts it, "You're the absolute best the U.S. Military has ever seen".
For Fletcher's part, he is disenchanted with lots of things dealing with the government but he takes Smith up on the offer because he "had nothing to lose. No family or friends to compromise me. I was utterly alone and though I'd never admit ti to Smith, though he probably already suspected it, I also desperately missed the action". Plus there was the six-figure salary plus any spoils of war he could acquire on his own.