Who is Princess Yvonne from The Story of the Trapp Family Singers and what is their importance?

Maria describes the scene when the Captain interrupts his proposal to the Princess to read Maria's registered letter to him. It is inferred that the Captain relayed these circumstances to Maria in order for her to be able to write about them. Princess Yvonne has never had the upper hand and immediately feels threatened by Maria and lets her know this soon after meeting her. One wonders who has the greater sense of pride here, the Princess, who was once on the verge of marrying the Baron, or Maria, who wins him and gets to tell the story.