The Master

The Master is a renegade Time Lord and archenemy of the Doctor.

Character History
The Master and the Doctor were once friends at the academy, but the Master's lust for power and desire to do evil made them part ways. After the Doctor was exiled to Earth, he did not expect to see the Master again. However, his enemy arrived on Earth with plans to ally himself with the Nestenes. When this plan fell through, the Master made many further attempts to ally himself with various alien species to try and gain power over the Earth. These were all foiled by the Doctor. At one stage, this incarnation of the Master allied himself with the Daleks to start a war between the Draconian and human empires.

The Master next appeared as a shrivelled husk after exhausting his regenerations. He sought the Eye of Harmony beneath the Panopticon on Gallifrey, which he believed would gain him a new regeneration cycle. The Doctor stopped him, but the Master evaded death. The Master would go on to try and gain himself new life by exploiting the Source on the planet of Traken. Though this plan was thwarted, he did get a new body by transferring his mind into the body of Tremas, a citizen of the planet. In this new body, the Master went on to be a thorn in the Doctor's side, frequently seeking a way of granting himself a new regeneration cycle.

The Master was finally killed by the Daleks as punishment for his crimes, but due to his experimentation into extending his life, he was able to survive as a snake creature that entered the mind of an ambulance driver named Bruce, granting him a new body. His plan to steal the Doctor's regenerations was foiled and he was sucked into the Eye of Harmony.

Other appearances
An image of the Master (presumably an early incarnation) appeared to the Doctor onboard the Nihil's TARDIS.

Personality
The image of the Master created by the Nihil exhibited a snide, taunting character, that took pleasure in the Doctor's frustration.

Season 1 (2015-2016)

 * 1) Devour (Illusion)