Who is Hephaistion from Time's Eye and what is their importance?

Alexander the Great's closest companion since they were boys, Hephaistion is a Macedonian who is taller than most. He has a long nose, with regular features set off by startling blue eyes. His hair is black and close cropped and he holds himself well, leaving no doubt that he is a warrior. Hephaistion has been Alexander's lover since adolescence, although the King has taken many wives, mistresses and other lovers, and is considered by the King to be his one true love of his life. As such, with a show of great faith, the King puts Hephaistion in charge of the army that is his advance group, making Hephaistion his "Chiliarch" or "Vizier" as he would be known in Persia. Hephaistion has the same deep love for the King, though taking many pages and other concubines to his bed while he and the King are apart.