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 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 affair
By the time that the 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. He sought to, for instance, 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 Sontaran invasion
After the Doctor willingly left Gallifrey again, in the Doctor's absence Borusa officially became Lord Chancellor (although the Doctor later claimed that had been done "illegally"). Although the Doctor returned to Gallifrey briefly to officially take up the post of Lord President, this was part of a strategy to defeat the Vardans. Once they and the Sontarans had been seen off, the Doctor now having been officially inaugurated as President of the High Council of Time Lords departed, again leaving Gallifrey in Borusa's hands. (DW: The Invasion of Time)

Career as Lord President
Sometime before the crisis with Omega, Borusa officially became instated as Lord President. (DW: Arc of Infinity). He used his presidential powers to investigate the legacy of Rassilon, from the Dark Time of Gallifrey. He gained access to the Tomb of Rassilon, where he believed he would find the secret of immortality, and thus be able to rule Gallifrey forever and 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, and tricked them into activating the secret transmat link that allowed him to join them there. However, the promised immortality proved to be a trap set by Rassilon to capture power-mongers such as he, and Borusa was condemned like several others before him to live his "immortal" life trapped within a sarcophagus. (DW: The Five Doctors)

The Doctor, incidentally, was immediately re-elected as Lord President by a grateful council and fled, leaving Chancellor Flavia with full powers until he returned. (DW: The Five Doctors)

Behind the scenes
Besides the Doctor and the Master, Borusa stands as the only Time Lord to be depicted in televised episodes through multiple regenerations (Romana only underwent one change on screen).