The Master

"The Master" — known in female form as "Missy", short for "Mistress", and at times by various other aliases — was a renegade Time Lord, originally a friend and long an opponent of the Doctor.

Though they had been friends from childhood (TV: The End of Time, Death in Heaven) and schoolmates at the Academy (TV: The Sea Devils, The Time Monster, The Five Doctors) before falling out, the Master developed an intense hatred for and often sought to kill the Doctor, who came to regard the Master as his arch-enemy. (TV: The Deadly Assassin, Castrovalva, Time-Flight, Doctor Who) Despite this enmity, however, the Master and the Doctor would on occasion act as allies. (TV: The Claws of Axos, The Five Doctors, The End of Time, The Magician's Apprentice/The Witch's Familiar)

The Master's diabolical madness was at least partially the result of a genuine malady in the form of a never-ending drumming sound that had been retroactively implanted inside his head by Rassilon and the High Council on the final day of the Last Great Time War to further their own goals. (TV: The End of Time).