Athelstan King is an agent with the British Secret Service.
The time period in which we first see the activities of this man in 1914, shortly after the country of his nationality entered what would be later called the Great War, aka World War I. The location of his adventures is the huge region called the Indian sub-continent. India has been under the control of British rule since 1857 though real dominance can be traced back nearly a hundred years before that.
King is said to be the seventh generation of Kings to serve and at times rule in India. His father, Cuthbert King, was a Raj-bahadur (as spelled by the chronicler, Rai Bahadur otherwise), "a title of honor bestowed during the British rule in India for faithful service or acts of public welfare to the Empire (according to Wikipedia)". It was said in one of the adventures that the elder King "was considered the equal of viceroys in all except rank, and greater than any viceroy in his grip on the hearts of men".
King is officially part of the British Army, holding the rank of captain but said to be "unattached" which I take to mean that he is not a part of any specific regiment, which makes sense being actually working for the Secret Service, but has the rank and position should it be necessary. His attitude in the military, though, is interesting in that he is respectful to senior officers as required but not overly so and definitely not worried about such things for himself.
Being born and raised in a land different from his nationality has given King a strange outlook. Even though he has spend a good number of years in schools back in England and wears the uniform of a British officer, he has strong feelings about India and deep down supports her desire for independence. It is not hard to imagine some of his colleagues being more than a bit confused by King's duo leanings. One acquaintance somewhat snarkily opined, "He's a believer in Kismet crossed on to Opportunity. Not sure he doesn't pray to Allah on the sly."
King seems to be a confirmed bachelor with no steady companion. He does, though, get thrown into extremely close quarters more than once with the beautiful, exotic, highly resourceful Princess Yasmini, "a law onto herself" in a remote region of the hilly northwest India. Yasmini, herself sometimes working with the British Secret Service but always for herself and her people, has entertained romantic thoughts about King but things just do not work out.
As time proceeds, King will give up his commission and his position with the Secret Service and go into work for an extremely rich man named Meldrum Strange. This individual, who lives up his last name, has some interesting ideas about how things should work in the world and has the money to see those ideas get a chance. All he needs are enterprising men to handle the leg work and he finds them in men like King and others ( and Jeff Ramsden are two such men). One interesting description of King can be found in one of those later adventures when a fellow point out, "Athelstan King - Englishman, but not half-bad; used to be a major in the army, but wears no monocle. Says, 'Haw, dontcherknow,' like the rest of 'em, but I'll say he's a scrapper if scars mean anything. Olive skin, burnt on from outside. Been to places."