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

Heather is a simple-minded girl, who tends the sheep at Ogion's home. She and Therru become very good friends while Therru and Tenar are staying at Ogion's, and Heather often takes Therru on walks through the meadows, during which time, she teaches her many things. Heather also allows Therru to watch over the goats with her, and Therru often finds herself chasing after an errant goat on Heather's behalf.