The Castellan was a Time Lord on the High Council during the Last Great Time War.
Biography
When the War Doctor returned to Gallifrey towards the end of the Time War, the Castellan aided Karlax in subjecting his companion Cinder to mind probe analysis. However, he grew uncomfortable with her treatment and hid her from Karlax behind Rassilon's portrait so she could recover.
The Castellan had his limits, and scruples with the Time Lords' methods, despite suggesting the use of the Tear of Isha against the Daleks in the Tantalus Spiral. When the Doctor tried to escape Gallifrey with Cinder and stop the use of the Tear, his TARDIS was prevented from take-off by the Time Lords having changed the relevant codes. The Castellan tried to persuade him to surrender, but the Doctor challenged him over the potential deaths of all the people in the Tantalus Spiral. Unwilling to be complicit in genocide, the Castellan transmitted the codes to the Doctor, enabling the TARDIS to dematerialise.
Karlax suspected that the Castellan had helped the Doctor escape and told Rassilon that he would investigate the matter. (PROSE: Engines of War)