Kenneth Ducane is the head of the British Secret Service.
At least he is the leader of a major department. The series never quite clarifies that but leaves no doubt that it is Ducane who decides which agent goes on which assignment. And on which double-agent or enemy agent never makes it home.
He is described uncharitably as "a very ugly man, of medium height, aged about fifty." The description goes to state his mouth too wide, lips too thin, nose a snub one with too wide of nostrils, and a shallow complexion that extends to his eyes. The general effect was frog-like. Indeed, he even has been nicknamed Froggie which his lack of vanity keeps him from resenting. But he is a frog with a good brain, very cool, very clear.
As the spymaster in the series, Ducane does not officially operate in the field any more, though he had done so for many years in the past. He dispatches operatives and then receives information back. He is shown to be fairly compassionate but still hard enough to send men to possibly die.
The third book in the series, Ministry of Death, has Kenneth Ducane semi-retired, changing his name, apparently, back to a true name of Kenneth Vandoran and leaving the Ducane moniker for his previous life. Why the change to Ducane had been made in the first place is not really explained. Later, though, in the fourth book mentioned here, Brock and the Defector, he returns using the Ducane identity. This fourth book is really in a mystery series about Inspector Brock but Ducane plays a key role so it is included here. And Ducane is Ducane again, even though the last book was published six years after Ducane's retirement.