Kestner is an agent with the American Secret Service.
Often in stories from just after the turn of the 20th century, that designation really means an unnamed agency inside the American intelligence community but in this case Kestner is actually a part of the real Secret Service. He acknowledges that he is a "roving agent for the Treasury Department". His membership in this compendium comes about because of that 'roving' part as well as the activities he gets involved with, especially in the second collection of adventures.
His position with the Treaury as an agent stationed in Europe was in no way "imperial. He had no actual jurisdiction over the territory he patrolled. He acted only under a tenuous extension of his own National Government's prerogatives. Beyond this, again, he was never permitted to act openly. He could never officially disclose himself, never personally purchase data or lay a charge or order an arrest He saw more inconvenience than romance in this system of preserving the full circle of anonymity. He was always face to face with his own limitations...He often thought of himself as a gardener set to watch a vegetable patch, warding off the hungry pullets without the power to wring their necks".
Kestner is described in the opening page of one of the recorded adventures as being a 'wandering mouchard' [def:an undercover investigator; a police spy, especially in a French-speaking country]. Sitting at an outdoor cafe and told to note a woman, he watched her "as his many years in the service had taught him always to watch his quarry, with that casual and intermittent glance, with that discreet obliquity, which could so easily be interpreted as the idle curiosity of an idle-minded sightseer".
Elsewhere we are told that Kestner "at times gave the appearance of being as lethargic as a blacksnake [but] could on occasions move with the astounding rapidity of that reptilious animal." He is known to apparently laze about as though he had no where else to be and then suddenly move with great speed and agility to accomplish a task without hesitation.
Kestner, as would be expected in a T-Man, is an expert in detecting forgeries, especially currency, but as we learn through his activities, he is well up on his knowledge of spy craft and the science of catching foreign agents. Interestingly, "faces were Kestner's penchant. He had what has been called 'a camera eye' for such things", all of which meant that if he had seen someone associated with a particular event, he could recall it perfectly years later.
We do not get to know anything really about Kestner outside of his work. This extends to never learning what his first name is. [Note: the synopsis of the movie mentioned below gives Kestner the first name of James but the name James does not appear in either of the books.]
In explanation as to why a description of a female suspect contained information about her ear lobe, "this was a feature to which the Department's Service always gave minute and scrupulous attention, for both the Secret Agent (Kestner) and the T D (Treasury Department) itself had long since leanred that an ear lobe is a most dependable short cut to identification."
"Nothing much mattered with Kestner. He felt that he had peeled life like an onion, layer by layer, and that in getting at its heart he had lost the onion itself. He had seen too much of things to respond to their surprises."
"New and mildly amusing hands were dealt out [by Fate to Kestner] but it was always from the same soiled old pack."
"[Kestner] abominated idleness."
"A Secret Agent, like a surgeon, should always work impersonally."