Borusa

Borusa was a notable and ultimately tragic figure in the history of the Time Lords of Gallifrey, and a very important figure in the life of the Doctor.

Earlier life and career
Borusa served as tutor to the First Doctor at the Time Lord Academy. (DW: The Deadly Assassin) As a junior Cardinal he chaired the Committee of Enquiry into the activities of Morbius. (PDA: Warmonger)

Involvement with the Master
By the time that the Fourth Doctor returned to Gallifrey to defeat the Master, Borusa occupied the post of Lord Cardinal on the High Council. He set about "adjusting truth" in the wake of these events. As part of this adjustment, he sought to portray the late Chancellor Goth not as the Master's ally, but as a hero who had given his life to stop the Master. (DW: The Deadly Assassin)

During the Sontaran invasion
After the Doctor willingly left Gallifrey again, in the Doctor's absence Borusa officially became Lord Chancellor, apparently illegally. The Doctor returned to Gallifrey briefly to officially take up the post of Lord President, as part of a strategy to defeat the Vardans. Once they and the Sontarans had been seen off, the Doctor departed, again leaving Gallifrey in Borusa's hands. (DW: The Invasion of Time)

Career as Lord President
Some time before the crisis with Omega, Borusa officially was instated as Lord President. Hedin left evidence of his crimes that made it appear that Borusa was the criminal. Borusa corrected the Time Lords, revealing Hedin's wrongdoing. (DW: Arc of Infinity). He used his presidential powers to investigate the legacy of Rassilon, from the Dark Time of Gallifrey. He pursued gaining access to the Tomb of Rassilon, where he believed he would find the secret of immortality - thus becoming able to rule Gallifrey forever. To this end, he reactivated the ancient Time Scoop, and revived the long-forbidden war games between captured representatives of various species in the Death Zone surrounding the Tomb.

He summoned several of the Doctor's incarnations to the Zone, knowing that at least one of them would gain access to the tomb. Once three of them had done so, he tricked them into allowing his transmat link to also access the tomb. Once there however, he found that the promised immortality proved to be a trap set by Rassilon to capture power-mongers such as him. Borusa was condemned, like several others before him, to live his "immortal" life trapped within a sarcophagus. (DW: The Five Doctors)

Revival and retirement
The Eighth Doctor persuaded Rassilon to free Borusa from his immortality to defend the Sixth Doctor at his trial. After Borusa did this, he was allowed to retire peacefully. (EDA: The Eight Doctors)

Behind the scenes

 * Along with the Doctor and the Master, Borusa stands as one of the only Time Lords to be depicted in televised episodes through multiple regenerations (Romana and Rassilon were each seen in only two incarnations on screen).