Borusa, 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 then became 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).