Who is Cal Hubbard from Harlot's Ghost and what is their importance?

Cal Hubbard is Harry's father and a highly respected member of the CIA. He and Harry's mother are divorced, and he is remarried to a woman by the name of Mary, with whom, he has twin boys. Mary leaves him at one point for cheating on her with the maid, but they eventually reconcile. When Harry is an adult, father and son work together on several projects and generally get along well. They correspond frequently through agency systems.