Who is Moss from Tehanu: The Last Book of Earthsea and what is their importance?

Moss is the witch of Re Albi, a village located near Ogion's home. Once standoffish and unkind to Tenar, she befriends her because they are no longer in competition with each another. Tenar even turns to Moss for advice on the relationships between men and women, as well as Ged's depression.