Spywatcher is an agent with the British Secret Service.
Sir Giles Prendergast is the head of the Secret Service that Spywatcher works for and it is from his lips we get an explanation, of sorts, as to why the codename of "Spywatcher" is mentioned above and not a first and last name. Sir Giles remarks, "Spywatcher has no name. No face either. Or, if you like, a thousand faces. Perhaps I've met him - and did not know it. Perhaps he lives next door to you. He is the most valuable counter-espionage agent in the country today - and I don't even know what he looks like!"
Communications with Spywatcher happens though a series of post office boxes with Spywatcher phoning from a call box, to avoid tracing the call, periodically to change the location of the post office. That said, on more than one occasion, Spywatcher actually talked via the telephone with Sir Giles so while the boss man's claim of not knowing what he looked like is likely valid, he had definitely heard his voice (and nothing was said about voice disguise).
Spywatcher often drives a family saloon (station wagon to us Yanks) which "looked just like a hundred thousand other rather dusty, slightly dented" other vehicles but only the man's service mechanic "knew what lay under the bonnet of the rather dull little car. Packed in, was a high-powered racing car engine of the latest design, with gear-box and transmission to match!"
We learn nothing about his life outside his career. We do not know if he has a family to go home to or what kind of abode that home might be. All we know is that Spywatcher is a dedicated pursuer of spies and leakers and other types of traitors.
The biggest facet of Spywatcher's repertoire, besides his incredible patience to sit and watch a suspect for hours on end, is his terrific abilities with disguises. The man is really quite adept at changing his appearance to assume a wide range of roles, each one going unchallenged. He uses the odd prosthetic such as nose builders and cheek plugs but he also achieves his anonymity through an impressive acting ability coupled with good use of dialects and mannerisms. He becomes who he want to be and makes the others see that person.
Spywatcher is human, a factor that makes him more entertaining and believable. He can get tired and frustrated when the hours and days of monitoring go by without a clue. He can make an assumption and be legitimately surprised when proven wrong. And some times his impersonations get him up close and personal with those he is stalking, actions which can and do put his life in peril.