Marcus Orlando is an agent with Department Y.
Department Y is a secret division in the British Intelligence community. We are told at the beginning of one adventure that it "has no politics. It serves whatever Government may be in power to the best of its not inconsiderable ability". The period of time for our following its activities is ostensibly the early 1930s but there is a feel to the tales that it really happens a decade earlier. There is intrigue abounding in the UK and in Europe but no hint yet of fascism. There is occasionally a question of whether monarchies should be allowed to continue or overthrown in different parts of the Continent and the fear of Bolsheviks on the one hand and just straight-out anarchists on another is displayed.
Orlando is likely the best operative that Department Y has and from descriptions, that organization has a fair number of agents working not only in the UK but throughout Europe keeping His Majesty's government officials aware of the situations. How long Orlando has been with the organization is not mentioned but we are given the feeling it has been some time. One thing for certain is that Orlando loves his job and would not think of changing professions - something he is given a chance to do at one point.
He has an "elaborate flat in Park Lane", made very comfortable through efforts of his devoted butler, Ivan Dolgourki and that man's wife, Nadja, Orlando's cherished cook. Orlando also has a Harlequin Dane named Bismarck he loves but who he will admit is one very dumb animal. We also learn that Orlando is wealthy enough to own a home beside a lake in the country where he loves to retreat to for solitude, taking Ivan and Nadja with him for comfort, of course.
Orlando is very good at his job. He keeps a steel card-index filled with secret information such as addresses, phone numbers, and other pertinent details, all this kept from inquisitive eyes by the use of Old Turkish script, extending so far as to encrypt it all with a code of his own creation. Also helping him in his work is considerable skill at disguises, able to assume identities to let him into any place with almost ease. When he takes on a role, he really takes it on.