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 longtime opponent of the Doctor.

Though they had been friends from childhood and schoolmates at the Academy, the Master's lust for power would eventually pull the two apart. While the Doctor and the Master would sometimes remain close friends despite their rivalry, the Master developed an intense hatred for and often sought to kill the Doctor, who came to regard the Master as their arch-enemy. Despite this enmity, however, the two would on occasion act as allies. The Tenth Doctor termed the Master as his "arch-frenemy".

The Doctor

"The Doctor" was the main alias used by a renegade Time Lord from Gallifrey who travelled through time and space with various companions in their obsolete and "borrowed" Type 40 TARDIS. They were the universe's "greatest defender", having saved the cosmos thousands of times throughout a long life, becoming a great legend across the whole universe.

Though largely a believer in non-violent conflict resolution, they were, when absolutely necessary, a great warrior. Indeed, some civilisations in the universe (e.g. the denizens of the Gamma Forests) translated the word doctor as warrior, whilst others saw the Doctor as a compassionate benefactor, worthy of their admiration and compassion.

Although they had saved untold numbers on their travels, the Doctor was thought to have caused the deaths of billions at the conclusion to the Last Great Time War. Though most of the Daleks were killed in the crossfire, Gallifrey - disappearing, rather than being burned - was hidden thanks to the efforts of "all thirteen" of their incarnations, the first eleven of which retained no memory of the event. For their actions, the Time Lords granted the Doctor a new cycle of regenerations, allowing them to live on after using up all available regenerations in their first cycle.

The Doctor's personal history was constantly changing and contradicting itself.