Kelner

Kelner served as Castellan of Gallifrey at the time that the Fourth Doctor returned to Gallifrey to claim the office of Lord President.

Biography
Unlike Kelner's predecessor Spandrell, he appeared to be much more political and ambitious. After the Vardan invasion, he assisted the Doctor in removing Time Lords who might oppose the new regime, drawing up lists of those he had a personal grudge against, such as Surgeon General Gomer. At this point he was told by the Doctor to consider himself as acting Vice President. Andred disliked him but stayed in post because he thought he might be able to lead a resistance movement within the Capitol and act as a foil to the corrupt Castellan.

Kelner was equally at home collaborating with the Sontarans who had used the Vardans as a cat's paw to their own invasion. (DW: The Invasion of Time)

Behind the scenes
It is possible that the unnamed Castellan in The Five Doctors is Kelner regenerated. As he states that the Doctor wants revenge, he may be referring to the events and his betrayal in The Invasion of Time. However, this would assume he was pardoned following his arrest at the end of this serial. This contradicts the Doctor's statement that he believes the Castellan was not a traitor. Although, if he was indeed Kelner regenerated he may have "reformed", but would still be under suspicion due to his actions in The Invasion of Time, making him the perfect scapegoat for Borusa.