Jack Whitfield is an agent with an unnamed organization.
We learn very, very little about the group of people who dictate which people they will send Whitfield to either eliminate or seriously mess with. Whitfield is as much in the dark as we are. He bemoans that fact numerous times but knows that initially at least all he knows about them is that their representative is named Simon and they are utterly ruthless, just ask Whitfield's wife, Anna.
Actually, you can't because in the very first couple of pages where we meet him, he walks into his home where he is quite happy with Anna and their five-year-old daughter, Maddie, only to discover the wife is dead from a slit throat and the daughter is missing. An incoming text informs him, "We have your daughter. Leave your house and get into the black car waiting for you outside. Do it now or your daughter will die." With that ominous statement, Whitfield is directed to a waiting car, has a bag put over his head, and then driven hours to an undisclosed location (in or around Atlanta [he peeked]).
Why Simon and his group picked Whitfield is not mentioned (as far as I could find) and what made him the candidate they just had to have so much so that they murder without hesitation and kidnap a child and then leap right into threats remains a mystery. He does not seem to ponder that oddity and we never hear him challenge anyone as to why him and why did his wife have to die and not get kidnapped like his daughter. He just accepts it.
Just as he accepts the fact that once at his secret destination, his life is altered even more with days and days of intense physical training and conditioning, lessons in a wide variety of fields including acquiring a convincing British accent. He even saw his body inked to a fantastic amount with "a one-eyed tattoo artist who jammed to Megadeth as he inked almost a third of my body". At each new twist in his forced metamorphosis, he is threatened repeatedly with harm to Maddie and a promise that should he complete his upcoming assignment, he would be reunited with her at last.
Of course, he does succeed and they do renege on their promise and there is yet another mission that he must do or she will be hurt and if he does everything will be peachy. Rinse and Repeat.