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. His later life gives us some fascinating insights into Gallifrey's political and governmental processes, as well as exposing the Doctor's frequent irresponsibility in such matters.

Biography
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)

By the time the Doctor returned to Gallifrey to defeat the Master, Borusa occupied the post of Cardinal in 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)

After the Doctor willingly left Gallifrey again, in the Doctor's absence Borusa officially became Lord Chancellor (although the Doctor later claimed this was 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)

Sometime before a crisis with Omega, Borusa officially became instated as 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 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)

In further twists on the matter of the presidency, the Doctor was immediately re-elected as president by a grateful council and fled, leaving Chancellor Flavia with full powers until he returned. (DW: The Five Doctors)

It is not known whether Borusa's trapped "immortality" allowed him to survive the later destruction of Gallifrey in that form. (DW: The End of the World, et al).

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