Who is Gwenhwyfar from Here Lies Arthur and what is their importance?

Gwenhwyfar is related to Maelwas and is said to be descended from or related to Ambrosius Aurelianus, a renowned war leader. When she is introduced in the novel she is married to Valerius the magistrate and protector of Aquae Sulis; as such, she is akin to the first lady of the city. When Valerius dies in combat Arthur claims not only his position as protector and magistrate, but also claims Gwenhwyfar as his wife. She is described as pale, very thin, and tall—features that gain her the nickname of Heron. Gwenhwyfar is reserved and finds Arthur repulsive. However, the marriage is a political arrangement and after a brief but transient approach by Arthur, Gwenhwyfar retires to a private life with her ladies in waiting.