Duke Douglas is an agent with American Intelligence.
Exactly which department he works for might be up in the air. At one point he reports to a big-wig in the Secret Service and does call him 'chief'. Another time he reports to another leader working out of the Pentagon. He is loaned at least once to the FBI for a mission and another time, the initial meeting we have of him, he is loaned out to the government of India by the Secretary of some unspecified cabinet office.
Whoever he answers to, and it is possible that since the adventures told of take place over a couple of years in the late 40s and early 50s it might have been more than one bureau, he has been around for quite a while and engaged in a good number of missions. We know that he got his start in the early days of WWII when he joined the Army (almost certainly as an officer since he was a college graduate) and was immediately assigned to the Intelligence (G2) branch. As the recorded adventures begin, he is likely in his mid to late 30s. This is based on his statement that he was a last year medical student when he joined Army Intelligence in 1942. That would have him probably 25 at the time he moved from the stethoscope to the cloak and dagger, 12 years before the series starts.
Douglas speaks and reads Urdu like a native. Other languages he is an expert with include German, Russian, and Spanish as evidenced by his ability to operate as citizens inside those regions without being noticed. Some of these he would have been trained in during the War. The others would have come later. There may be others we do not learn of. He is an excellent shot, very dangerous in hand-to-hand combat, and knows which end of the knife to hold in a duel. He is also a skilled pilot and unafraid to take chances in the air or on the ground (shown by a willingness to land a small plane in the Alps).
A quite good looking blond haired man (maybe with a tint of red in it), Douglas is attracted to the ladies and they are most definitely drawn to him. He probably has sworn off long term relationships, though because of the incident with the beautiful and deadly LLyanya Davany, a Soviet agent he worked with during WWII when Russia was an ally and he was ordered to cooperate. He did more than get along and fell in love. When some intelligence he foolish passed to her ended up with her bosses and gave Stalin an advantage in negotiations with the other Allies, he knew she had betrayed him and the relationship ended. A decade later when running into her on an assignment, the old feelings returned as did the distrust.
Douglas' areas of expertise are impressive. Europe, the Indian subcontinent, the Middle East, and Central and South America are places he has and can walk with confidence and know he has a chance of survival, a chance people who challenge him are in short supply of.