Mason Walker is an agent with Onyx.
He will be, at least. Shortly after we first meet him he will be offered a position with that organization and will accept it. Before that, he is a civilian trying his best to raise his daughter, Clara, now nearing high school graduation, as he believes his late wife, Bree would have wanted.
Gone for some years now are the days he spent as one of the most elite fighting forces on the planet, the U.S. Navy SEALS. He gave up the action and the excitement, both things he knows he enjoyed, to devote his time to something a lot closer to home and while he does miss the being a part of something bigger, he has no regrets over his decision.
Before he could walk away from the SEALS, however, there was still the matter of five years remaining in his enlistment. Because of his exceptional capabilities and his performance to date, Walker was offered a special placement. His CO put it, "We have a shadow team within the SEALs. Not even the other SEALs know about it. We'd like for you to join. Less years, better pay, earlier retirement". Walker took the deal and then spent three very full years conducting missions all over the globe.
Now the story of Walker jumps ahead fifteen years later to the year 2028. In that time, Walker's wife, Bree, had given birth to Clara, had a bad bout with cancer, beaten it, and then, a few years later succumbed to a powerful return of the disease. Walker had fulfilled a promise to his wife, stayed out of danger, and raised their daughter to be the fine young woman she is when we meet them.
That is when trouble from his distant path will come back to cause trouble for the both of them and the only way he could find to save them was to seek help from old friends. This help would result in an offer to join Onyx.
One aspect of Walker that will make him both an exceptional operative and a major pain in the backside of whoever has to try to lead him is that Walker, as his superiors in the SEALS put it, had a "tendency to freelance [which] would someday put his men in a precarious spot". By that they meant that "compared to the rest of them, Mason could rise above the fray, make a critical decision within a millisecond, and execute upon that, even if it came at the risk of 'mission protocol'. While such a role was common amongst leaders back at the base, for a soldier to possess it on the ground in as quick a manner as Mason had left many dumbfounded - and infuriated when he went off schedule. Mason always called it his superpower, much to the chagrin of his superiors, but it always worked out for him. ... Because not only was Mason Walker tactically sound, proficient in all types of firearms, and a useful soldier, he was a quick thinker, an improviser of the highest degree, and creative to a fault. He could make any commanding officer flinch from afar, but at the end of the day, it usually worked".
Note: one oddity I have not figured out is that Walker is called 'Sergeant' in one adventure and refers to one of his SEAL teammates as his former 'Sergeant'. The SEALS are in the Navy. There are no sergeants in the Navy.
"There was no such thing as a calm mission for a Navy SEAL - and if it was calm, it wasn't a mission, it was a task."