Kelner

Kelner served as Castellan of Gallifrey at the time that the Fourth Doctor returned to Gallifrey to claim the office of Lord President, when the planet was invaded by the Vardans and later the Sontarans. The Doctor described him as a "jumped-up guard".

Biography
Unlike his predecessor Spandrell, Kelner 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. Although Andred disliked Kelner, he 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. (TV: The Invasion of Time)

Behind the scenes
It is possible that the unnamed Castellan in Arc of Infinity and The Five Doctors is Kelner regenerated, but this seems unlikely given his arrest at the end of this serial. It is important to note that there is no evidence to support this, other than the title of Castellan, which has been bestowed upon several Time Lords.