The Master
The Master, like the Doctor, is a Time Lord. He's typically been presented as a Moriaty figure. The character first appeared in the Jon Pertwee story Terror of the Autons in 1971. The Master featured in every story that season.
Memorably created by the actor Roger Delgado on many occasions during the third Doctor's period, the character had a period of absence following the Roger Delgado's early death shortly after Frontier in Space.
The Master returned in a degenerate body in the Tom Baker tales Deadly Assassin and Keeper of Traken. In the 1980s, the Master appeared regularly played by Anthony Ainley.
The last story broadcast prior to Doctor Who's cancellation in 1989, Survival, included a memorable 'final battle' between the Master and the Doctor.
Unfortunately, the misconceived TV Movie of 1996, revived the Master in overtly camp fashion, played this time by Eric Roberts.