Thomas Thornhill is an agent with GLASS.
Glass is a branch of the American intelligence community, apparently an amalgam of people from the CIA, NSA, and FBI. Its headquarters is an "underground fortress in Detroit, a hidden contribution to the city's recent rebirth". It is run by Mr. Crumwell, a thin old man with scant white hair and blue eyes. Exactly what the official reason for the existence of Glass is, I do not know. The 'real' reason is a desire to go up against The Twelve, a cabal made up of a dozen very powerful men that the people behind Glass feel very much need to be brought down. Since we really do not learn that much about Glass, we do not learn a whole lot about The Twelve except that they are very powerful and very ruthless.
We also do not learn a great deal about Thornhill before he was accepted into working for Glass. We know that he was a Marine sniper who had "the average-looking body of a healthy man, a body that gave no hint that it had gone through marine physical training with flying colors, unofficially besting the record time at the hardest marine obstacle course in the world".
What we do learn is that Thornhill is, while very much a rookie in the spy business, a very resourceful and determined man who has a sharp brain behind his average looks and the grit needed to continue with the mission when everything has gone wrong. It does and he does.
There is also the interesting woman who works as Thornhill's handler, Mallory, someone who becomes a whole lot closer to him. She will prove to be more of an incentive to Thornhill than even his sense of duty.