Slade is an agent with the KGB.
He is a double-agent working in British Intelligence but really working for the Soviets. The two-book series about this unpleasant, short, chubby traitor with a large unpleasant face, whose first name is never revealed, features two different agents for the Secret Service of England but the series is definitely about Slade.
Not much is ever known about Slade. Why he betrayed his country is not known, nor is it ever discovered whether he was a plant by the Soviets from the beginning or did he grow hungry and sell out for money.
What is known is he is a treacherous, deadly man who has no qualms about killing whoever gets in his way, even his colleagues and superiors. He is extremely devious, clever to a fault, and possessing an intuition that keeps him alive in the strangest of situations. He knows that the men he twists about and forces to do his dirty work could easily kill him but he knows the right buttons to keep them in line. And it is difficult to believe that, though he works for the Soviets, not even they are pushing any buttons on Slade.
In the first book Slade is very much in control while in the second he is far less dominating but never less dangerous. His keen intellect and a very good memory makes him a font of information which makes him very valuable and, since he can use almost any situation to his advantage, just when he looks at his most vulnerable, he can find a way to sneak out a victory.
Note: The second book, The Freedom Trap aka Mackintosh Man, was made into a very good but very gray movie by the latter name, directed by John Huston, and starring Paul Newman. The movie came out in 1973 and starred Ian Bannen as the deliciously devious Slade.
Note: The first book, Running Blind, was turned into a three-part television miniseries by the BBC in 1979 and starred Stuart Wilsom as Alan Stewart with the role of Slade being performed by George Sewell.