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 Tenth Doctor deactivated the chip, Waechter mutated. The Doctor and Martha Jones left him on Agelaos with the rest of the mutated humans.