Ash Carter is a private investigator.
That's the best I can come up with and it is not really accurate but then again nothing else I tried was accurate either. He is not an adventurer although he does quite a bit of travelling and having adventures though not by his choosing. He seems to be available for being 'invited' by governmental officials to help in this little matter or that but he is not officially working for any organization. Mostly he is just a man in the wrong place at the right time and there is no word for that.
We meet Carter first doing this unofficial private investigating but then some time later we are given a trio of adventures from before that time, back when he was in the military, stationed in Palestine as part of the Royal Military Police, Special Investigations Branch. He is 24 at that time. There he had a bit of authority and could detain people and conduct interrogations and make arrests. He was good at it, despite his relative young age. He was also good at poking his nose into things some powerful people wanted ignored and that led to trouble and that would lead to a desire to get out of the Middle East post haste and a request for help from an old friend in Singapore was very timely.
Carter stands 6'2 and is in impressive shape, such that when a couple of miscreants decide, as foolish people do on more than one occasion with him, to cause him bodily harm, Carter is very capable of dissuading them in a violent manner. His having been a promising boxer in his teens and someone who still enjoys the training regimen helps him stay in shape and also gives him an advantage in a fight; he instinctively knows what an opponent is about to do.
The adventures that Carter gets into all tend to be in the mystery/police investigation/private eye parts of thrillers which bleed over into the espionage field with terrorists and would-be government overthrows and the like.