Who is Prince Sigmund from Without Feathers and what is their importance?

Prince Sigmund is a character in a ballet who falls in love with Yvette, a woman who is cursed by the magician Von Epps to exist as half human, half swan. Sigmund, however, is engaged to someone named Justine. He first agrees to save Yvette, then decides to marry Justine, and finally leaves her when Yvette is found.