Nardole

Nardole is introduced in the 2015 Christmas special "The Husbands of River Song". He was hired by Professor River Song as a servant on Mendorax Dellora on Christmas Day 5343. He unwittingly brought the Doctor and River back together, mistaking the Doctor for a surgeon River had requested to extract a diamond from the head of the ruthless King Hydroflax. During the episode Nardole had his head cut off, but he managed to survive because it was uploaded to King Hydroflax' cyborg body.

In the 2016 Christmas special "The Return of Doctor Mysterio" Nardole appeared as the Doctor's companion. Once his final night with River Song had concluded, the Doctor was somehow able to reattach Nardole's head to his body, Nardole joining the Doctor as a companion so that the grieving Time Lord would have someone to stop him going too far over his grief at his final parting with River.