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

Cunaide is the wife of Arthur's youth. She is described as beautiful and desirable and apparently she enjoys drinking, carousing, and sexual exploits with Arthur. When Arthur goes to war, Cunaide remains at home. When Arthur conquers Aquae Sulis and marries Gwenhwyfar, Cunaide remains at her distant home. Myrddin explains that Arthur and Cunaide, married under Pagan law, are not legal spouses and therefore need no divorce prior to his subsequent Christian (and therefore legal) marriage to Gwenhwyfar.