Jerricho

This unnamed Time Lord held the post of Castellan for some time during the Doctor's fifth incarnation.

Biography
He was responsible for matters relating to security on Gallifrey, and answered to Lord President Borusa. This Castellan was on Gallifrey during Omega's attempt to harness the Arc of Infinity. (DW: Arc of Infinity)

During the Death Zone crisis, the Castellan was among the High Council members who received the Master on Gallifrey, in order to send him into the Zone. The Castellan gave the Master a transmat device with which he could return to the Capitol. When the Doctor and Master were ambushed by Cybermen, the Doctor used the transmat device to escape. Wondering how the Cybermen found him so quickly, the Doctor found a homing beacon inside the transmat device.

President Borusa immediately ordered a search of the Castellan's rooms, which turned up a chest containing the Black Scrolls of Rassilon. Borusa ordered the Castellan to be interrogated using the mind probe, but the Castellan was soon shot by Gallifreyan guards, apparently after trying to escape. In truth, Borusa was behind the Death Zone crisis, and had seemingly framed the Castellan. (DW: The Five Doctors)

Behind the scenes

 * It is possible that this Castellan is Kelner from The Invasion of Time, having regenerated. However, the Doctor's impression that he is blameless puts this theory under some strain in light of Kelner's actions in Invasion.