Zeb Carter is an agent with the Agency.
This small government organization is not to be confused with the CIA, also known often as the Agency. It is run by a sharp and dangerous woman named Clare who holds the 'nebulous' title of Director of Strategy, a person who put together the department to 'undertake exothermic missions that no other Special Ops or deep black ops agency in the country's defense and intelligence setup could or would undertake'. By exotermic she means high risk and/or high danger and all with the need for total deniability should something go wrong. It often means doing things which are officially illegal, like taking out a foreign head of state, or super-secret, like retrieving biological weapons that should not even exist.
Apparently the Agency had at one time been a bit larger than it currently is but when Clare was handed the reins, she overhauled it to make it totally deniable and that meant a good deal smaller; the smaller the footprint, the harder to trace it. That does not mean, however, that she does not carry a considerable amount of power in the intelligence community because taking on assignments other do not want often means amassing a good deal of favors as well as a reputation for getting positive results.
Chief among those getting those results is Zeb Carter, a man she met through a close college friend. Having dinner with that friend one night, she observed a man who was waiting outside the establishment who 'seemed to become part of the street'. The friend told her that man was her brother, a 'superhero'. After she learned more about him by looking into the official files on him, she began to agree with the description. Carter had been Special Forces for many years before leaving and working a short while as a mercenary then becoming a security consultant. Carter worked within his own set of rules which included never going against women or children or taking on work that went against his country's best interest.
When Clare first approached Carter to work for her, he surprised with by turning it down but then counteroffering. He and his select group of associates would be available for any job she might have but would also be free to take on other clients to keep sharp and to maintain their independence and their deniability. Her acceptance was, she always considered, one of her best decisions.
Carter is extremely taciturn and seldom smiles. Having lost a wife and child years before, he is highly reluctant to get romantically close to anyone but he has gathered a close band of friends, each highly gifted in the 'security' work and each as devoted to Carter as he is to them. Carter by himself is almost unbeatable. With his friends, he is dynamite.