Will Parker is an agent with American Intelligence.
Since the adventures we have of this man take place in modern times, calling who he works for as something so nebulous seems odd but then again he really does not work for any one group.
When we first meet him, Parker is practically a civilian and has been one for a few years, working as district attorney in a small-ish county in Georgia. He still enjoys it, to a degree, but as he ruminates, putting away the same offenders for a few years only to see them repeat their offenses shortly after release got old. He was looking for a change and a chance for some excitement even if he did not consciously know it.
I mentioned his civilian status as being 'practically' and by that I mean he still served as a reservist in the Marine Corps, holding the rank of colonel which he had earned at a fairly young age. Even now when we meet he is only 42 years old and still in impressive shape due to a fairly rigorous training regimen and a whole lot of running, including marathons. While an active Marine, Parker had been part of the highly prestigious Force Recon and then followed a series of very specialized training including a good deal learning all he could with the Navy SEALs.
One other important, and very vital skill that Parker possesses is an incredible gift for languages. He was compared at one point to the legendary Cardinal Mezzofanfi, a man who spoke 20+ languages fluently. While Parker does not enjoy quite that many on his resume, he does have a lot and even more impressive is a perfection when it comes to dialects and accents. While his Caucasian feature would prevent him physically passing for, say, Korean, to just hear him would be good enough to convince the listener.
The adventures we have of Parker have him working in different parks of the globe which means his training in different climes will be a big factor in survival and his talent with languages will be used a lot.