Jack Crockett is agent with Homeland Security.
Well, maybe. He is associated with them for much of the time we follow his actions but he is known by, and called upon by, the U.S. Chief Executive so perhaps it would be better to say he was a Presidential Agent. Then again, he is said, rightly or wrongly, to be a freelance agent - even a mercenary - so the matter does get muddied.
One thing not so murky is the fact that Crockett - and that is how he prefers to be called - is an imposing figure both to those he goes up against and to many he is trying to help. First, he is "tall and powerfully built", something that helped him make a name for himself in the NFL back before he abruptly left sports and went to work soon after for the government. He has jet black hair "cut short, though longer than military grade". "His face was rugged but not leathery, attractive but not quite handsome. His nose bent at a slight angle. With his musculature and air of confidence, he would have been sexy even without the eye patch". That patch was over his left eye, "a souvenir of another long-ago battle"; his one remaining visible eye was an alluring turquoise. To the woman who was making these observations, he "projected an air of contained violence", and she was no stranger herself to violence.
Regarding his days on the professional gridiron, it is said that he became a bit annoyed at a linebacker on an opposing team who had beat up his girlfriend and bragged that no one could touch him. Crockett proved him wrong in the next game by breaking the man's leg, intentionally. He quit the NFL the next day.
Crockett then joined the military, went on to become a Special Forces operator, served for a few years and then left. Some reports had him becoming a mercenary but the truth seems to be that he became a weapon made available as needed to whatever department had use of his impressive talents for getting the job done, regardless of how many limbs (of others) he might have to break.