The Master (Doctor Who) performer Anthony Ainley
The Master, or "Missy" in their female incarnation, is a recurring character in the British science fiction television series Doctor Who and its associated spin-off works. They are a renegade alien Time Lord and the childhood friend and later archenemy of the title character, the Doctor. They were most recently portrayed by Sacha Dhawan. 
Multiple actors have played the Master since the character's introduction in 1971. Within the show's narrative, the change in actors and subsequent change of the character's appearance is sometimes explained as the Master taking possession of other characters' bodies or as a consequence of regeneration, which is a biological attribute that allows Time Lords to survive fatal injuries or old age.
The Master was originally played by Roger Delgado from 1971 until his death in 1973. The role was subsequently played by Peter Pratt, Geoffrey Beevers, and Anthony Ainley, with Ainley reprising the role regularly through the 1980s until the series’s cancellation  in 1989. Eric Roberts took on the role for the 1996 Doctor Who TV film. Since the show's revival in 2005, the Master has been portrayed by Derek Jacobi, John Simm, Michelle Gomez, and Sacha Dhawan.
Beevers, Roberts, Jacobi, Simm, and Gomez have reprised the role in audio dramas produced by Big Finish Productions. At the same time, Alex Macqueen, Gina McKee, Mark Gatiss, James Dreyfus, and Milo Parker portrayed incarnations unique to Big Finish.A new regeneration cycle – the long serving Master
Played by Peter Pratt in his next appearance, the Master returns in The Deadly Assassin (1976), opposite the Fourth Doctor (Tom Baker). Special effects makeup was applied to Pratt to give the Master a corpse-like appearance. Found by Chancellor Goth (Bernard Horsfall) on the planet Tersurus, the Master is revealed to be in his final regeneration and near the end of his final life. The Master attempts to gain a new regeneration cycle by using the artefacts of Rassilon, the symbols of the President of the Council of Time Lords, to manipulate the Eye of Harmony at the cost of Gallifrey. But the Doctor stops the Master, who escapes after his assumed death.The Master later returns in The Keeper of Traken, the role taken over by Geoffrey Beevers. Still dying, the Master came to the Traken Union to renew his life by using the empire's technological Source. Though the plot fails, the Master manages to cheat death by transferring his essence into the body of a Traken scientist named Tremas (Anthony Ainley) and overwriting his host's mind.The Master (Ainley, in a total of 31 episodes as the character) would confront the next three regenerations of the Doctor on and off for the rest of the classic series, still seeking to extend his life – preferably with a new set of regenerations. Subsequently, in The Five Doctors (1983), the Time Lords offer the Master a new regeneration cycle in exchange for his help.The Master's final appearance in the classic series is in Survival (1989, the final story of the series' original 26-year run), trapped on the planet of the Cheetah People and under its influence, which drives its victims to savagery. Though the Master manages to escape the doomed planet, he ends up back on the planet prior to its destruction when he attempts to kill the Seventh Doctor (Sylvester McCoy).