J.J. Stoner is an agent with an unnamed British Intelligence organization.
We initially meet him through a series of short adventures, the first one detailing how he had been a sergeant in the British army, a soldier who had been around for some time which makes Stoner certainly in his early 30s at least. The incident in question took place in Iraq during the conflict there and the unfolding story told how Stoner had sought information from recently captured combatants after a skirmish which resulted in one or more of Stoner's men being killed. Stoner asked questions with a tone which "was one of weariness, resignation, no particular aggression, and certainly no violent intent". When he got no answers, he killed one of the captives and continued asking the others until there were none left.
This obviously did not sit well with the upper command when word got out, and word always gets out. Stoner's career in the military was certainly over and he stood a very good chance of facing war crimes trial. That is until "Hard Man", as Stoner thought of him, showed up and offered a different outcome. This new path would entail working for him and his off-books group doing what Stoner has recently showed himself good at - killing with no emotion or compunction.
As time progresses, Stoner's work will change a tad and sometimes his dispatching of a person or two would not be exactly under orders. Doing his hunting of people and eliminating them all without a uniform took a bit of getting used to but Stoner is a man used to taking orders and if that is what he is told to do, so be it.
Later when his job description is changed and he is asked to not be the one doing the killing but instead going on the hunt for other killers, well that will take a bit of getting used to.
But one thing that will remain is that by and large, when Stoner does what he does best, which is engage in violent activities, he really does it well.