Christopher was the twin brother of Robert.
Christopher died from cancer when he was eight years old. Robert, having spent several years time travelling in a flawed time machine, was returned to 1966 by the First Doctor using the Fast return switch, where he took Christopher's place.
Unable to treat Christopher's illness, the Doctor took Christopher to Mars before he died, allowing him to see the Earth from space and witness something remarkable in his life. (PROSE: The Little Drummer Boy)