Who is Harlot (Hugh Montague) from Harlot's Ghost and what is their importance?

Hugh Montague, also known as Harlot, is Harry's godfather. He holds a high position in the CIA, and he was directly involved in the overthrow of the Guatemalan government. Harlot later becomes deeply involved in the plot to get rid of Castro. He is described as a terse man with many enemies, but Harry both respects him and wants to impress him.