Etienne Réhmy & Gaston de Blanchegarde are agents with the French Secret Intelligence, Permanent Staff.
They are by no means the only characters of note in this series but they are the ones actively employed as operatives. The other two of great interest, Thomas Preston and the diabolical Professor Anselm Kreutzemark, have other occupations.
Réhmy is for certain an agent for his government's intelligence service. De Blanchegarde is, in the first adventure, described as being with the French Flying Corps, 'temporarily attached' but listening to the history of these two, that attachment has been going for several years and show no signs of ending. They have been working as a team since likely well into the First World War and there is no indication they plan to stop any time soon.
Réhmy is described as the "cool, competent brain of that curious partnership". He is a "short dark man between thirty and thirty-five" who was "of the Southern French type with the brownest eyes I have ever seen, eager and sanguine of expression". The last part is shown on numerous occasions as he, when not deeply involved in deadly work, is prone to be a happy, cheerful fellow, the kind people like to sit and have a drink with.
de Blanchegarde is described as "tall and dark. He had a thin kindly face with an indiscriminate sandy moustache cut close and almost level with the ends of his mouth." He had the "eyes and habit of a student" while also having "downcast, stooping manner of the overtrained official." In the partnership, he was "its impulsive and occasionally rash executive". A definite man of action, he chafes when forced to sit and wait, often bounding from a seated position to pace excitedly about a room for a short while before sitting back down in frustration. When it comes time to pounce on an adversary, however, his energy comes in handy a lot.
Narrating the two adventures and playing a vital role in each is an Englishman named Thomas Preston who works as a representative for his Uncle James of Jebbut & Jebbut, hardware merchants, otherwise known as a traveling salesman. Described by himself as being of fair hair with blue eyes, 5'11", Preston has been sent prior to WWI to study at university in Bonn where he learned during his two years there to speak German like a native. He was to have gone on to Oxford when the war broke out so he joined the army instead and rose to the rank of Captain in the Royal Field Artillery, receiving two wounds and the Military Cross. Preston does not have the training in cloak and dagger work that Réhmy and de Blanchegarde have but he does not lack for intelligence and perseverence, not to mention good old stubborness. Of course in the first adventure he has the heart of a beautiful woman to win, not to mention saving her life at least once, but in the second there is compelling reason for his joining the escapade except his determination to stop a really bad guy.
Professor Kreutzemark is the bad guy and he does an excellent job in that role. [Note: in some lists of series, it is he that is the named character, not Réhmy or de Blanchegarde nor even Preston though he be the point of view for both tales.] The Professor, as he is called throughout the stories, is "a figure notorious in the archives of every intelligence service in Europe". He is said to be "one of the finest chemists in Germany and responsible for a dozen foul inventions. He it was who discovered Yellow Cross gas".
The Professor is a man of considerable reputation in the back rooms and alleyways of intrigue. Use of his name can bring recalcitrant fellows to heel and make even stalwart brigands take a pause. When he is angered, which he can be at times though he hides it very well behind a thin curtain of civility, he can with but his sharp tongue and his great control of himself take charge and bring his sense of order.
Kreutzemark is described as quite tall and thin. He has a golden beard that is, while not odd looking, is nevertheless distinctive. More important to his whole coutenance are his eyes. They are extremely piercing, missing nothing as he slowly looks about a room. His ability to read people's emotions is frightening and his mesmerizing glare can and does hypnotize through his look and his melodic voice. Adding to an already impressive presentation, Kreutzemark travels with a red parrot of "malevolent appearance" who sometimes squawks "I spy strangers!" followed by a piercing whistle. According to Preston, it is very disconcerting.
With two experienced and dedicated intelligence operatives, one intelligent and resourceful amateur, and one extremely dangerous, vicious, heartless antagonist, this is wonderfully crowded arena.