Cardinal of the Time Lords and one of the most powerful figures in
their history. He was once the Doctor's (and presumably the
Master's, the Rani's and Drax's) teacher at the Time Lord Academy.
According to the Doctor, he supposedly said "Only in mathematics
will you find the truth". A consummate politician, Borusa
was, and undoubtedly always had been, one of the major movers in
Gallifreyan politics. He was very much concerned by the
proprieties, legalities and appearances of things. When the
Fourth Doctor saved Gallifrey from the Master and Chancellor Goth,
Borusa ordered Castellan Spandrell to prepare a cover-up story for the
general population (4P). Soon afterwards, a regenerated Borusa
became Chancellor and helped the Fourth Doctor when Gallifrey was
invaded by the Vardans and the Sontarans. He came to express a
grudging respect for his former pupil and surrendered the true Great
Key of Rassilon, which enabled the Doctor to tap the power of Harmony
to power the Demat Gun. He was then made President of the High
Council of Gallifrey in place of the Doctor (4Z).
However, when Omega threatened to return to our universe by taking over the Fifth Doctor's body, a once-again regenerated Borusa did not let his sentiments interfere and voted to condemn the Doctor to death (6E). Borusa regenerated again (which may have unbalanced his mind) and fell victim to a megalomaniacal passion. He decided he wanted to rule forever and acquire the true immortality once promised by Rassilon. In order to break through the Tomb of Rassilon, he used the Five Doctors and their companions, pitting them against various threats in the Death Zone. Rassilon finally gave him the immortality he sought by turning him into a living statue (6K).