Nardole

Nardole is a small, fussy, oddly capable man who turns up as an assistant with a habit of getting reassembled after mishaps. By the time he settles alongside the Doctor, he's become a self-appointed keeper of promises and routines, nagging, worrying, and refusing to let his friend cut corners on a very long vigil.

Matt Lucas plays him for comic exasperation first, which makes the moments he turns serious land harder — a joke of a man who turns out to have a spine. The writing uses him as the show's conscience-with-a-mop, and Lucas never lets the sentiment curdle into sap.