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. He was a Lord President, Cardinal and Chancellor of Gallifrey (the last position, arguably, having been assumed illegally).

EarlY life and career
Borusa was a 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
When the Fourth Doctor returned to Gallifrey to defeat the Master, Borusa was Lord Cardinal on the High Council. He set about "adjusting truth" in the wake of these events. He portrayed 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, 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 the Sontarans and they 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 was inducted officially as Lord President. Hedin left evidence of his own crimes that made it seem Borusa was the criminal. Borusa corrected the Time Lords, revealing Hedin's wrongdoing. (DW: Arc of Infinity)

Descending Into a Madman
He used his presidential powers to investigate the legacy of Rassilon from the Dark Time of Gallifrey. He pursued access to the Tomb of Rassilon, where he believed he would find the secret of immortality, ruling Gallifrey forever as 'President Eternal'. 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.

Borusa summoned all of the Doctor's incarnations to the Zone (though the Fourth Doctor ended up becoming trapped in the Time Vortex), convinced that at least one would gain entry to the Tomb. Once three of them had, he tricked them into allowing his transmat link to also access the Tomb. After reaching the Tomb, however, he found that the promised immortality was a trap set by Rassilon to capture power-mongers such as himself. Borusa was condemned, like 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. The second incarnation of Borusa was released to carry out this duty, after which he was allowed to retire peacefully. (EDA: The Eight Doctors)

Personality
Borusa seemed to be a strict teacher towards the Doctor. He didn't give the Doctor much credit for saving Gallifrey from the Master. However, the pair seemed to have a love/hate relationship, only for their relationship to completely shatter when Borusa placed all of the Dotor's incarnations in mortal danger.

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, alongside Romana and Rassilon who had two incarnations on screen.
 * According to REF: The Brilliant Book 2012, Borusa's Portrait of Rassilon was stolen during his tenure as Lord President. The Corsair, a renegade Time Lord, was accused and censured for it.