Razor (World Enough and Time)

"Mr Razor" was an alias and persona kept up by for at least ten years while he was stranded aboard a Mondasian colony ship. Bill Potts lived with Razor for ten years, believing him to be a close friend, although the Master, after eventually dropping the disguise, would go on to claim that he had found her "so boring for all those years", only keeping up the pretence for the sake of eventually hurting her and thereby getting at the Twelfth Doctor.

The Razor persona was an eccentric character with a long, disheveled hair and beard, a thick if undefined Eastern European accent, and dark, crumpled clothing.

Background
After being "kicked out" of Gallifrey, landed on a Mondasian colony ship which was experiencing time dilation due to pulling itself away from a black hole, and took over the city on Floor 1056, where he "lived like a king until they rebelled against [his] cruelty". Attempting to escape, but being "too close to the event horizon", the Master burned out his TARDIS's dematerialisation circuit, stranding him on the colony ship. (TV: The Doctor Falls)

Activities
Disguising himself as "Razor", the Master pulled the strings of the "genesis of the Cybermen" with Operation Exodus, working for a hospital hosting the Conversion Theatre. "Razor" found Bill Potts, who had been given a cybernetic chestpiece after being shot on Floor 0000. Understanding that Bill was from 21st century Earth, the Master found it necessary to maintain his disguise, acknowledging that he was her "former Prime Minister." (TV: World Enough and Time) The two of them spent ten years on the lower decks, (TV: The Doctor Falls) where the Master learned that the Twelfth Doctor was at the front of the ship. Studying the woman travelling with the Doctor,, the Master eventually deduced that she was a future incarnation of himself trying to turn good. Becoming "concerned about [his] future", the Master lured Bill into surgery for full cyber-conversion, knowing that the Doctor would never forgive him for it. Watching as the Doctor, Nardole, and Missy arrived, the Master revealed his identity to Missy, and the two of them gloated to the Doctor about the fate of his companion. (TV: World Enough and Time)

Legacy
Out of his disguise, briefly adopted Razor's manner to taunt the converted Bill Potts. (TV: The Doctor Falls)