Jackson is a freelance agent.
Not really but then again a one or two word description is not coming to me.
When we first meet him, he is working for SO12 which apparently is some designation for a South African Secret Service. We are told that though he is an operative in good standing with them, he holds British citizenship. It is likely therefore that it is a dual-citizenship then that we are talking about. It also seems a bit a of a moot point as he is that southern African nation at the beginning of the first recorded adventure and then no more as he is ordered on a mission that will take him out of that country back to England and from there all sorts of places except South Africa. Plus he seems to be asked to do these chores for the British Special Branch and for the Canadian Intelligence community and for the CIA - it gets a wee bit confusing.
Jackson thinks of himself as an Intelligence officer trained to get classified information from others in a clandestine manner; he does not consider his line of work to be eliminations and yet that is all we find him doing; I do not know what to make of that. However he might peg his line of work, when it comes to killing, he is extremely good at it; he is efficient and quick and repeatedly successful.
He also has paid for his reputation in two ways. The first is visible in a very noticeable scar on his face, thick and white running "from his left eye down the side of his cheek almost to his mouth"; it has caused him to "grow out of the habit of smiling". The second is internal as he tells us in one instance that "it hadn't even occurred to him not to kill [his target]. Had he left his morals behind when he'd joined the Secret Service, or had it been before that, in his previous life? He couldn't remember."
At one point (it is a bit confusing to me) Jackson becomes married to Jennifer Lynley, a woman that I would definitely not want to cross but whom I would like to have as a partner; she is very efficient herself and very daunting.