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. At various points in his life, he assumed the ranks of Lord President, Cardinal and Chancellor of Gallifrey.
Biography
Early Life and Career
Like the Doctor, Borusa was a member of the Prydonian Chapter. (DW: The Deadly Assassin)
Borusa held the degree of a jurist and was a tutor to the First Doctor, (DW: The Deadly Assassin) the Master and the rest of the Deca at the Time Lord Academy. (MOV: Doctor Who: Genesis)
Borusa became increasingly frustrated with the Doctor's poor grades. (MOV: Doctor Who: Genesis)
Involvment with the Master
When the Doctor returned to Gallifrey in his fourth incarnation, he was framed by the Master and Chancellor Goth for the assassination of the outgoing President. By this time, Borusa was Lord Cardinal on the High Council. During his trial, the Doctor proclaimed his candidacy for the President as a delaying tactic. Once the Doctor was shown to be innocent, Borusa set about "adjusting the truth" in the wake of the events, portraying the late Goth not as the Master's ally, but as a hero who had given his life to stop the Master. Unsurprisingly, the Doctor was appalled and was more than happy to leave Gallifrey in return for the charges against him being dropped. (DW: The Deadly Assassin)
During the Sontaran Invasion
After the Doctor had left Gallifrey once more, Borusa became Lord Chancellor — illegally, according to the Doctor — and at some point regenerated. (DW: The Invasion of Time)
The Doctor later returned to Gallifrey briefly to claim the post of Lord President, based on his previous candidacy for the position. The Doctor was secretly working to defeat the telepathic Vardans in their quest to invade Gallifrey, a ploy which he eventually confided to Borusa. The Doctor asked his old mentor to remain in a lead-lined room to prevent the Vardans from reading his mind. Borusa acknowledged that he was too one-track-minded to employ the Doctor's 'trick' of keeping his thoughts too chaotic for the Vardans to read. Once the Vardans and their puppet masters, 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
Sometime before the second crisis involving Omega, Borusa was inducted officially as Lord President and regenerated again. Councillor Hedin left evidence of his own crimes that made it seem that Borusa was the criminal. Borusa corrected the Time Lords, revealing Hedin's wrongdoing, and hoped that Omega would find peace after the Fifth Doctor sent him back to the antimatter universe. (DW: Arc of Infinity)
Descending into madness
Regenerating yet again — with his stubborn streak apparently enhanced in the process — Borusa 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 previous 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. He also arranged for the Master to be sent into the Death Zone- officially to help the Doctor but unofficially to 'inspire' the Doctor by giving him a clearer villain to face- and time-scooped at least one Dalek, one Robot Yeti, and a squad of Cybermen to go into the Zone as well. Although the Fifth Doctor was sent to the Capitol and managed to learn what Borusa was doing despite his attempts to frame other parties, Borusa used the Coronet of Rassilon to take control of the Fifth Doctor once the first three Doctors had reached Rassilon's tomb and been tricked into deactivating the forcefield blocking transmat access to the Tomb. After reaching the Tomb, Borusa used the Coronet to paralyse the Doctors' assembled companions, but the first three Doctors were able to link minds to free their fifth self, the four Doctors joining together to stand against Borusa. As the spirit of Rassilon appeared in the tomb, he offered Borusa immortality, but as the First Doctor had realised, 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)
Legacy
When a virus caused all the personality copies of all living Time Lords to escape from the Matrix, Borusa's Matrix copy ended up transplanted inside the Rani, partially bringing him back to life. Borusa attempted to kill the Rogue in order to become Lord President again but was thwarted when the recently cured Rogue administered the cure, causing the Matrix copy to return to the Matrix. (PAN: Maybe It's Imaginary)