Jack Strong was the son of Harroll Strong and the twin brother of Dot Strong. He had a dead-white complexion and red hair.
Jack accompanied his father and sister on an Earth-Mars expedition to Corbo, during which he and Dot were the only ones not captured by the devil-birds. The First Doctor found them and, after rescuing the spaceship crew, took them to the TARDIS. (PROSE: The Devil-Birds of Corbo) Before being returned to Earth, they had at least one other adventure with him when they travelled to Rhoos. (PROSE: The Playthings of Fo)