What is the name of the Doctor's arch-enemy, a renegade Time Lord?

The Doctor’s arch-enemy, a renegade Time Lord, is the Master. Like the Doctor, the Master comes from Gallifrey and can travel through time and space, but the two use their extraordinary abilities for radically different purposes.

The character first appeared in 1971 during the Third Doctor story “Terror of the Autons,” played by Roger Delgado. The Master repeatedly sought power, conquest and control, often manipulating governments, alien civilizations and other villains rather than relying only on direct combat.

The Master’s ability to regenerate has allowed several actors to play different incarnations, including Anthony Ainley, Eric Roberts, Derek Jacobi, John Simm and Michelle Gomez. A frequent mix-up is the Rani, another renegade Time Lady, or Omega, a powerful Time Lord figure from the Doctor’s past. Neither is the Doctor’s defining recurring arch-enemy.