Jack Russell is an agent with American Intelligence.
Naturally there is no specific agency with that name and refers to the intelligence community as a whole but that is, at this time, the most I can give because almost nothing else is given us. We know by several inferences that he is American and we can surmise by his actions that he is in the clandestine business but beyond that, nothing.
Russell is described as looking "like one of the new breed of CEOs - he wore a white shirt and khaki trousers, with no tie and no jacket". He has muscular arms and bronzed skin, so bronzed it looks almost burnt. "His eyes sparkled like diamonds as they caught the light" but they can also be "cold as steel".
While we do not know how old Russell is, but his reaction to things around him we must conclude he has been in the cloak and dagger business for at least a decade. He is too calm and determined to be less experienced - he is not blase about the situations he finds himself in but neither is he surprised or anxious. By the same token, he is not likely to be much older than say mid-30's because of the athleticism he must produce to handle a couple of particularly strenuous events.
Strong and dependable is how Russell comes across. In a couple of the problems he gets thrown (literally more than once) he shows a doggedness that is admirable. His body is screaming at him to lay down and die but his mind refuses to do so because, well, it isn't in his nature to just give up. And in one case, he is standing in the open with death racing at him and little chance of survival and he doesn't panic or quit even though there is virtually no chance of making it out but still, virtually doesn't mean actually.