"The Master" — known in one incarnation as "Missy" (short for "Mistress"), in another as "the Lumiat", and at times by various other aliases — was a renegade Time Lord, a power-hungry "lover of chaos" who served as both a friend and the arch nemesis of the Doctor.

Friends from childhood and schoolmates at the Time Lord Academy, the divide between the Master's lust for power and the Doctor's empathy for "lesser species" would eventually pull the two farther and farther apart — to the point that the Master often sought to kill the Doctor. Despite this enmity, however, the two would on occasion act as allies, and both continued to yearn for their old friendship.

Like the Doctor, the Master fled from Gallifrey in a TARDIS of his own, and, having fully embraced his darker nature, would go on to pit himself against the Third Doctor and UNIT during the Doctor's exile on Earth. Later, having expended his original regeneration cycle, the Master survived in the ghastly form of a living cadaver, in which form he fought the Fourth Doctor, before exploiting the powers of the Source on Traken to steal the body of Tremas. He would continue his crusade to submit the universe to his will in a variety of stolen or otherwise fraudulent bodies, from using Tzun nanites in order to gain new regenerations to transferring his essence into a Deathworm Morphant, which allowed him to survive execution by the Dalek Prelature, and continue to survive by possessing a succession of human bodies.

The Master has powers and skills like that of any Timelord but additionally, he has the abilities of Genius-level intelligence, the ability to regenerate, the use of a tissue compression eliminator, perception filter, hypnotism, disguise, and natural charm and charisma. For a bit of time temporarily, he even had the ability to shoot lightning out of his hands.

He is a known renegade criminal of Gallifrey, having committed crimes of Genocide, Theft, Breaking and entering, Abuse of power, Mass murder, Torture, Assault, Subjecting people to a fate worse than death, Incrimination, War crimes, Kidnapping, and much more.

Physically, The Master is 5 foot 7 with a broad and toned muscular body build, dark brown hair with tints of grey, and he has brown colored eyes. He has fair-toned skin and facial hair of a mustache and stubble along his chin and the bottom half of his cheeks.