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

Therru is merely a child, a young girl, who was discovered burned by a group of tramps. Tenar is called on to help heal the child, but finds herself unable to do much more than keep Therru comfortable. Over time, Therru heals, but the remaining scars leave her an outcast among other children. Tenar adopts Therru, and she takes her with her when she visits Ogion during the final moments of his life.

Therru is a great source of joy for Tenar, and when Tenar and Ged are taken under the control of the wizard, Aspen, Therru proves to be their salvation. Unbeknownst to everyone, she has the power to call a dragon who comes to save Tenar and Ged. In the end, we learn that Therru is one of a rare group of people who are dragons, who live as humans in order to learn all they can.