Who is King Philip of Spain from The Virgin's Lover and what is their importance?

King Philip of Spain is encountered first as Queen Mary's husband before Elizabeth comes to the throne. Old Queen Mary chooses King Philip so that her reign will be better accepted by the old families at court, protect England against the French, and further cement their joint Catholic cause. King Philip is depicted as a grand and powerful ruler, and somewhat of an opportunist. As the King of Spain and the Netherlands, he hopes to add England to his collection of realms through a successful marriage with Queen Mary hopefully bearing a child.