Brett Carstairs is an agent with British Intelligence.
According to Jess Nevins' awesome The Encyclopedia of Pulp Heroes, the Online Edition, he is "a handsome member of the British upper classes who lounges around Monte Carlo and similar places playing sports, enjoying himself, and pretending to be 'woman-proof'. However, that is a ruse. He's actually a very active member of the 'Secret Corps working on behalf of his country's Intelligence Department', carrying out counterintelligence missions against the Soviets.
He is also, in the first recorded adventure, very much in love with a lovely young woman who is the step-daughter of a prominent British government official. Carstairs' problem in this case is not only might the step-father actually be betraying his country to a seductive Russian woman, the step-daughter and love interest of Carstairs might also be carrying on with people from the Soviet Union. Young Carstairs is then placed in a horrible position of having to possibly get the woman of his affections locked up for treason!