Sydenham is an agent with British Intelligence.
If asked officially, he will gladly, and proudly, that he is attached to the Home Office which implies, rightly that he has some measure of clout but which also conveniently masks what he actually does by saying a lot without telling anything.
Sydenham - if he has a first name I have yet to find it - would never be close to anyone's description or concept of an operative. He in fact would be almost the antithesis of a cool and dynamic secret agent and yet he not only is one, he is quite good at it.
He is described early on in the first recorded adventure as being "no Adonis". This is explained with:
"Men and women who met him for the first time were wont to thank God for their own good fortune, for he was a parody of the body beautiful so much admired today by the human race. Bald head and far, round shoulders joined in a single curving outline that seemed to dispense with the need for any supporting neck. He was hairless, with not even a monk's fringe of relief above his small, flat ears: nostrils and ear-holes were likewise devoid of any hirsute growth, and only a redness, a deepening of skin colour, showed where his eyebrows out to have been. Strict dieting had failed to bring his weight below seventeen stones so that his stomach bulged over enormous thighs, to give him a kind of Toby Jug squatness. His back was broad, and spoke of considerable strength, but this was challenged by the smallness of his hands."
All this negative "unattractiveness" is countered, however, by a very keen brain and a truly compulsive inclination to dress not only properly but also well and to carry himself with a correctness and forthrightness that might seem polite to many but which brooked little opposition.
Sydenham worked, as mentioned, for the Home Office but his work invariably was on the side of national security and he was very, very good at it.