Who is Guilliadun from The Lais of Marie de France and what is their importance?

Guilliadun appears in Eliduc. She is the fair maiden who captures the heart of Eliduc. Unaware that he is married, she faints as if dead when she discovers he is. She is later revived by Eliduc's wife, who is so taken by her beauty that she becomes a nun and allows Eliduc to keep his mistress.