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

Elen de Quincy is Llewleyn and Joanna's daughter. She is unhappily married (arranged marriage) to John the Scot, and later, begins an affair with Robert de Quincy. Elen and Rob's affair continues throughout her marraige, and after her husband's death many years later, the couple marries.