The primary character in The Human Factor is Maurice Castle, an aging agent with the British intelligence service who is leaking information about South Africa to the Communists. Castle is leaking information out of gratitude to his friend, Carson, a Marxist who rescued Castle's eventual wife from prison. His choice to leak this information leads to a chain of events, beginning with the killing of his coworker, who is wrongly believed to be the double agent.

The supporting characters you may include in an analysis of The Human Factor are Sarah Castle, Arthur Davis, Dr. Emmanuel Percival, Cornelius Muller, Colonel Daintry, and Sir John Hargreaves. Sarah Castle is Maurice Castle's wife. She met Maurice Castle when he was a field agent in South Africa, and when Arthur Davis dies, she immediately suspects that the British were behind it. Arthur Davis was Maurice Castle's coworker and was wrongly suspected of being the agent who was leaking governmental secrets to the Communists. He is therefore killed on the recommendation of Dr. Emmanuel Percival, a member of British intelligence who is summoned by Sir John Hargreaves to deal with the security leak. He feels no remorse when he is given the news that the wrong man was killed.

Cornelius Muller is another important character. A representative of apartheid, Muller opposed Castle seven years before the book's events when Castle broke South Africa's race laws. Muller eventually suspects Castle of being the double agent and informs Hargreaves. Colonel Daintry works below Hargreaves, and his humanity leads to difficult situations, especially when Davis is wrongfully killed. When Daintry visits Castle to discuss Davis' death and Castle admits that Davis was not the double agent, Daintry reports this to his superiors with the intention of resigning afterward. A discussion of the book's characters may also include Sir John Hargreaves, Daintry's superior. Hargreaves is the one who instructs the security officers that they must eliminate the double agent. Because he places so much trust in intuition, when Muller suspects Castle, Hargreaves reluctantly sends Daintry to question him.