Jason King is an agent with Black Force.
Black Force is an unofficial American military operation, conceived and created by upper echelon officers, "top of the food chain" as King refers them, to take on the missions that they felt needed to be done but which few in their right mind would attempt and certainly none would admit to. As a completely off-book organization but financed through funneling from legitimate projects, it sent its men all over the world to handle "things that never would have ordinarily been sanctioned".
King is likely around 30 when first we encounter him. As he explains it, he joined the army at a young age and within a couple of years had made it into Delta Force when he was just reaching legal drinking age. "Everyone else hated me. I was at least five years younger than any of them. They accelerated me through the ranks quicker than most." The reason for the rapid advancement was that "in training drills I was noticed by a few higher-ups. They watched me. Ran some tests. Turns out I have close to the fastest reaction speed on earth."
After just a couple of years in Delta, these men of the nascent Black Force approached King to be their first operative and he, looking for excitement and challenges, agreed.
"There's zero evidence that anything I did ever happened. They sent me into the worst hellholes on earth. Often alone. I was their freak science project. I saved a lot of good people, killed a lot of bad people. I was Black Force's main operative for eight years. I don't know how many others there were. No-one told me anything. I was a ghost, a secret independent contractor. They paid me enormous sums of money to do the things that never would have ordinarily been sanctioned."
Then, two months before the first recorded adventure, "everything was beginning to catch up to me, and I couldn't take much more of it.' Not the killing so much though that was part of it. "It was how close I kept coming to death, over and over and over again. I knew it wasn't sustainable. Sooner or later, I'd be too slow. I'd get caught from behind. I'd find myself in open ground, with no cover. Something like that. It ate away at me until I finally mustered up the nerve to call it quits."
So he quit. Told his bosses he was done and left. That is when we first meet him as he is wandering the world looking for something he cannot quite name. Life will throw things at him, including an eventual return to Black Force, and he will react as he always does, decisively and often permanently.