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.
Biography
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 crimes that made it seem 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 access to the Tomb of Rassilon, where he believed he would find the secret of immortality, 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. (DW: The Five Doctors)
During his tensure as President, Boura's Portrait of Rassilon was stolen. The Corsair was aqussed and censured for it. (REF: The Brilliant Book 2012)
Borusa summoned several of the Doctor's incarnations to the Zone, convinced 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. After Borusa did this, he was allowed to retire peacefully. (EDA: The Eight Doctors)