Steve Walker is an agent with the State Department.
Career people in that section of the government are usually refered to as diplomats or words in that vein but since Walker belongs to the Bureau of Intelligence and Research, the term 'agent' more than applies. It is obvious from his talk that he, and his colleagues, put themselves in the same category (okay, a cut or three above) as the CIA so again 'agent' is appropriate.
Walker's area of expertise at State is the Middle East which makes a great deal of sense because until he was six, his parents lived in Tehran and Walker has a native's grasp of Farsi, speaking it like a true Tehrani and not just someone taught it at a langauge school. And while he had not been back to Iran in all those years, his knowledge of and understanding of that part of the globe is impressive. Throw in a better than average skill in Arabic and you have the makings of a valuable asset.
Also add to the mix a secret he has kept from most people and that is that he has a half-brother named Azrak (according to him, his mother's indiscretion and his father's excuse to drink) who has in the past few years risen to a position of some authority in the Irani hierarchy.
Walker is a widower who lost his wife to cancer some time before the first recorded adventure. As we see numerous times in the escapades of this man, his love of her remains high but so does his normal male libido so when opportunity with the fairer sex knocks, he is quick to open the door.