Who is Elphin Rheged from A String in the Harp and what is their importance?

Elphin Rheged was chosen by Taliesin to become the next bard. He is described as a gangly, near-grown boy, brown and careless. Over the course of the novel, Elphin ages in the key's vision and is later kidnapped by pirates who want to use him for his labor. He is never seen or heard from again.