Who is Harry from The Constant Princess and what is their importance?

Harry is Arthur's younger brother and second son of Henry Tudor. He is described as young and impetuous. Arthur tells Catalina that Harry believes himself in love with her, something that eventually clears the way for their marriage.

Although Harry's devotion to Catalina is evident, his interests are always subject to change. When England is faces a double threat—France on the one border and Scotland on another—Catalina sends Harry off to fight France, all while planning to command the defense against Scotland. It's noted that Catalina recognizes his lack of focus, both as a king and a husband. She learns early on that her husband requires entertainment, and she takes on the task of finding something new to capture his interest on a regular basis. Sadly, Harry eventually loses interest in his wife, and Catalina finds herself defending her right to the throne.