Clay Loomis is an agent with the CIA.
Loomis had left West Texas to join the Marines at 15. He fit in well and became a model soldier, tough and ready to fight when challenged, as his oft-broken nose would testify. A 6' 3" and well built, he was always able to see the other guy ended up worse.
One of officers he served under convinced him to get his GED and then attend college, which he did. After earning his B.A., he got commissioned and was stationed at Gitmo.
After his tour was up, he stayed in Cuba. He had met some of the rebels fighting for Castro and liked their earnestness and their desires to remove the corrupt dictator Batista. He was helping them when he was approached by same officer who had guided him before, a man who now worked at Langley.
Recruited by the Agency, Loomis was around when Castro turned the island communist. He was assigned to help train for the Bay of Pigs and, when that turned out a disaster, he got transferred to Indochina. Vietnam, Laos, Cambodia, and other places saw his work but he quickly grew angry with the things the Agency was doing and tried to stop some of them. In response, the CIA tried to terminate his employment, with extreme prejudice. Twice.
Since then he has worked as a mercenary for a time before taking a job in the Dominican Republic organizing their police and armed forces. There is where the series opens.
Clay Loomis is an aging expert in matters of combat, killing, and staying alive. Just shy of 50 years of age, though, can make a person start to worry about the future, if you are lucky enough to have one. He has a well-paying job as advisor to El Jefe in that island nation and, while there are many rumblings of discontent, he is moderately content.
That is when, with the permission of the U.S. President, the Agency comes to him with an offer. All would be forgiven if he would come back into the fold to help with a dangerous situation. Loomis must decide if he is willing to put his aging neck on the line again.