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

The Lady of Caerwent appears in Yonec. She is the unnamed lover of Muldumarec and the mother of Yonec. Once beautiful, her beauty fades after years of being locked away by her husband. When she discovers love again, her beauty returns, betraying her affair. She becomes pregnant by the knight, and gives birth to Yonec. Later, she reveals the story of his father to her son, spurring Yonec to kill her husband in revenge.