Jack Noble is a freelance agent.
He is a contract killer for hire with the people doing the hiring usually intelligence agencies who need someone removed without it getting out they were behind the hit. The operative word here is usually because he also works on the odd job for others. There was no decision on his part on day to become an assassin but when circumstances forced his hand, he showed he was very, very good at it.
When his desire to be a football player did not work out, he joined the Marine Corps. This might not have been his smartest moves as in Basic Training he showed his ability to tick off just about everyone. Many of his fellow recruits at Parris Island took a dislike to him, probably because he was very good at everything. All of his instructors disliked him because he could take what they dished out and still smile. It looked very quickly like military life and he would not be a match. That's when the CIA stepped in.
The powers-that-be had an idea of using Marines in conjunction with CIA operatives in hotspots all of the world. They needed men who could do more than just take care of themselves but they also wanted people who were not totally set in the ways of the military. The solution to this need was to look at the new trainees and pick out the ones that look promising. Further training at their own facilities would tell if the prospects measured up. If not, they were sent back to the Service without trouble. Jack Noble made it. So did his new-found friend, a giant of a man nicknamed Bear.
For nearly a decade, Noble and Bear worked all over the globe backing up the Company's men. Then 9-11 happened and they were sent into Iraq ahead of the troops and someone somewhere got the idea that these two would watch an innocent family get massacred - or perhaps they were setting Noble and Bear up. Whatever the cause, the actions that the two took to try and save the people got them on someone's naughty list, a list whose members usually died. Noble and Bear did not.
After the trouble that got them so unpopular was over, Bear and Noble went parted on friendly terms and Noble got a job working an American Intelligence group called the SIS (not the same as the British MI6). A couple more years of action around the world would pass before Noble decided he needed to part ways with the SIS and reunite with Bear to form their own company. Contracts came in once it became known how good they were. Soon a fair number of groups and governmental agencies wanted their help. A fair number wanted their heads on a platter. Sometimes the two groups were the same.
As the series evolves, Noble will work for several different "employers". His allegiances will never vary, though. He is loyal to his friends. Everyone else just pay the rent.