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

Lark lives in Middle Valley and is Tenar's neighbor. She is described as a kindly woman, who has many children. When Therru is burned, someone goes to tell Lark of the injured child. Lark immediately collects the child, and then runs to Tenar to ask for help. Therru recovers from her wounds in Lark's home, before she is finally adopted by Tenar.