Waechter

 was a human colonist from the planet Agelaos. He was the last human on the planet and was tasked with keeping the beacon, which warned people away from the planet, in check. The only thing keeping Waechter alive was the chip in his neck which also kept him from ageing. When the Doctor deactivated the chip, Waetcher mutated. The Doctor and Martha leave him on Agelaos with the rest of the mutated humans.