Who is Elen de Quincy from Falls the Shadow and what is their importance?

Elen de Quincy is Llewleyn Fawr and Joanna's daughter. She marries John the Scot, which is considered an advantageous match at the time. Elen is unhappy in her marriage, and thus, she begins an affair with Robert de Quincy..... an affair that lasts many years. When Elen's husband dies of spotted fever, she quickly takes the opportunity to marry Rob, bringing shame upon her family.