Dennis Tyler is the Chief of the CPI.
That is the acronym used for the Bureau of Current Political Intelligence. This is a division of the State Department. Its description in one of the recorded adventures of its leader tells not only a great deal about the bureau but also, by extension, about its leader:
"The C.P.I. had been the bane and blessing of three Administrations. Its work was neither obviously necessary nor easily dispensable. Its dozen members formed a close-knot group in State Department politics and bureaucratic intrigues and were to be found mainly at the messier end of the earth, getting at the heart of a revolution in the banana belt, heading off arms shipments in Mongolia and Sonora, watching the slow and terrible chess-moves of the great battle of gold and steel and wheat in Eastern Europe, or plunging into the heart of Central Asia on 'scientific expeditions' which caused head-scratching and heart-searching in a half dozen world capitals.
"Sometimes the C.P.I. was in favor, sometimes in disgrace... Most of the time, however, the Bureau and its Chief were regarded as a sort of necessary evil. Dennis Tyler was generally lucky and, what was more important, officials who tried to get rid of his services were apt to be unfortunate."
About Tyler, it was said often and in different ways that he was "a young diplomat whose hopes of promotion lay in the quarrels of the nations and the squabbles of his superiors". That does not paint a particularly nice image of the man, though it is usually true. What was not said in such statements was that when his services were in need, his ability to pull off the required miracle was highly impressive. Since he did work in an incredibly unorthodox manner, though, the Establishment invariably resented his intrusion, at least up until their rears were in the fire and needed extracting at which point it was 'hail and well done'.
There are frequent references to Tyler being a 'young man'. This is certainly a matter of relative age because, while there is no stated year in which any of the adventures take place, they were definitely over a decade after the end of WWI. There are numerous referrals back to people that Tyler met as a young officer during the Great War, sometimes being assigned as a liaison. Guessing that Tyler would have been at least 25 around then, he is at least 35 when the first of recorded adventures occur.
Tyler is happily married to Cynthia who indulges his several instances of sudden trips abroad with interesting aplomb - she is not shown to be surprised or upset at his skedaddling but does, according to him, treat him fondly upon his return. Since numerous times someone refers to him as a 'red-haired dandy', it would be curious to know if his sartorial style is really worthy of being commented on and, if so, is it he that choses his dress or his wife.
Tyler's membership in this collection comes because each of his several missions invariably throw him up against agents of one sort or another of foreign governments. He especially has to deal more than a few times with Bolshevik intrusions, sometimes they are quite real and dangerous and sometimes they are merely perceived and feared dangerous. Often there is the need to investigate a murder with political ramifications.
Regardless of whether it is finding out who killed the odd diplomat or courier here or there or who absconded with what secret document, from Tyler's reaction to such events, it is clear that solving these matters is what that man lives for. The stranger the problem, the stickier the wicket, the more Tyler is in his element.