David Gibson was an American. His family visited Wales when he was a child. Years later, when he and his friend Jack were hitchhiking through Wales, they became involved in the evacuation of Tír na n-Óg.
When Nuada summoned his demonic brethren to invade Earth, David was drawn to him because he had been possessed by a demon when he was there when he was young. David killed Nuada, after which the Seventh Doctor managed to remove the demon from him. (PROSE: Cat's Cradle: Witch Mark)