Secret Agent B2-3 is an agent with American Intelligence.
I am pretty sure that is a true statement but there is so little known about this operative that it is only a guess as for whom he works. In the three recorded adventures available, nowhere is his nationality or affiliation actually stated.
In the first mission we find of his, he is in Tibet trying to prevent a would-be world conqueror from conquering the world. Who sent him is never mentioned. At the time we think the assignment took place, everyone was pretty busy at World War II. The second job has him definitely in the States based on the architecture and the fact everyone is driving on the right side of the road. The third and final assignment, though, told a good year after the first two, has him in hat and trenchcoat talking to a British Admiral, Sir James Dunwoodie, who greets him by saying he "had to borrow you from the Air Intelligence". Whose military, British or American? It is not said.
So I put him down as American working for their intelligence community but I could be wrong.
His ability to blend in while in disguise is darn good. He takes the role of a High Lama in the mountains of Tibet and either he pays the other lamas really well to go along with his fakery or he speaks Tibetan well enough to fool the natives. When he poses as inventor Jim Carson, everyone at the factory and in the neighborhood is convinced which is also an accomplishment, unless his name really is Carson and he does invent planes on the side.
He is depicted as a good looking white man in his late 20s or early 30s. He has a good relationship with the opposite sex but perhaps his relationships do not last. The beautiful Manya is his love interest in the first case. He is working closely with a Mary Lynn in the second (she is definitely American so likely B2-3 is as well). There are no women in sight in the third one.