Jack Killer is an agent with Britain's SIS.
Getting what amounts to an overview of his dossier from the first recorded adventure, we learn that 'Jack Killer' is just the codename he was assigned at the Secret Intelligence Service (MI-6). His real name, which he never uses, has been redacted in the files. He hails originally from Brazil but most likely was raised in the U.K. because he acts decidedly British. He has an impressively high security clearance, which is no surprise consider much of what he does is so incredibly covert, the fewer who know about it, the better. The cases we have of his take place in or around 1974 and have been only recently unsealed.
Killer has a remarkable resemblance to the late American actor, Charles Bronson. Like Bronson, while his appearance would never be considered handsome, it definitely has a rugged appeal and when coupled with his innate physical ability and his aggressive demeanor, it works to make him very attractive to women. He is frequently seen having to leave a beautiful woman behind to head to a mission or is able to find equally gorgeous ladies while on that mission, or both.
It is said several times that Killer does not work for SIS proper but for a "black ops section" run by a sometimes unpleasant "fat man named Alpha", a fellow who thinks nothing of calling Killer at any time expecting immediate response and snarling when not getting it. Killer takes this attitude in stride, likely knowing that since he is undoubtedly Alpha's go-to guy, he has the latitude to shrug off such complaints.
A couple of interesting observations so far:
While flying from London to New York, a city that Killer had been to several times before and enjoyed a lot, he muses while sitting in coach that "At least James Bond gets to fly first class".
While lamenting with his boss about the nasty things he is forced to do on the job, Alpha reminds him
that's the nature of our work, Jack. It's our duty." To which Killer responds sadly, "No, Alpha, it's my life. I just wish I didn't have to be so bloody good at it."