Adelphi

Adelphi of the Arcalian Chapter was a councillor on Pandad IV's High Council of the Time Lords (PROSE: The Legacy of Gallifrey) and a member of the tribunal that sentenced the War Lord and the Second Doctor during their own separate trials (TV: The War Games) alongside Socra and Goth. (PROSE: The Legacy of Gallifrey) He later came to Earth on several occasions to give the Doctor with warnings. (TV: Terror of the Autons, PROSE: An Overture Too Early, etc.) On one such occasion, he warned the exiled Third Doctor that, a renegade Time Lord and old friend of the Doctor, was on Earth. (TV: Terror of the Autons)

Biography
Adelphi, along with the Time Lords Socra and Goth, had captured the War Lord. The he explained what the War Lord and his people had done. He said that the War Lord's plans were "an utterly callous disregard for the lives of the humans involved, and for the liberties of all the other species in the galaxy." When he asked the War Lord if he had anything to say in his defence, the War Lord refused to say anything. Adelphi used some of his power to cause intense pain for the War Lord, thus he spoke, saying that the Doctor collaborated with the War Chief and that if he was guilty then the Doctor was guilty too.

Adelphi sentenced the War Lord to dematerialisation, wiping him out of existence after his verdict of guilty was reached. The second Time Lord listened to the Doctor's defence when his own trial came up and listened as the Doctor passionately addressed the Time Lord's own non-intervention policy and saying they should be helping those in need much like he did. The Time Lord later witnessed the Doctor refuse every new appearance offered to him when he and Goth announced he would be exiled to Earth. (TV: The War Games)

Adelphi was a member of the High Council, in addition to serving on the tribunal that sentenced the Doctor. PROSE: Doctor Who and the Terror of the Autons) He had been sent to the tribunal on orders of Lord President Pandad IV as a direct result of Rassilon's agent, Socra, being asked by his Matrix Lord masters to keep an eye on the Doctor. (PROSE: The Legacy of Gallifrey)

He later served as a messenger to the Doctor in another incarnation. In this capacity, he first encountered the Third Doctor when he appeared in his TARDIS while the Doctor was with his companion Liz Shaw. (AUDIO: Shadow of the Past) After the Doctor's defeat of Helios, the messenger spoke to the wardens of Shada, requesting that they release and keep the Doctor busy on Earth. (PROSE: Prisoners of the Sun) He then travelled to Earth to warn the Doctor of the Master's arrival and of an impending trap that the Master had set for the Doctor. When the Doctor asked for assistance in dealing with the trap, however, the messenger simply disappeared. (TV: Terror of the Autons) He was sent to see the Doctor again in order to persuade the Doctor not to travel with a future companion whom the Doctor had not yet taken on board the TARDIS. (PROSE: An Overture Too Early)

Behind the scenes

 * Adelphi first appeared in the 1969 serial The War Games as an unnamed Time Lord Councillor played by Trevor Martin, who was credited as "Time Lord 2". He appeared again in 1971's Terror of the Autons as a Time Lord messenger played by David Garth. Terrance Dicks' novelisation of Terror of the Autons revealed that the messenger had been part of the tribunal that sentenced the Doctor in The War Games. The character finally received a name in The Legacy of Gallifrey by Gary Russell, a 1975 short story in Doctor Who Magazine 100 that provided an in-universe history of Gallifrey and the Time Lords. The Legacy of Gallifrey named the three members of the tribunal as Goth, Socra and Adelphi. Because it explicitly established both Goth and Socra as separate from the Time Lord messenger, The Legacy of Gallifrey confirmed that Adelphi and the messenger were the same individual.
 * Some time after he played Adelphi, Trevor Martin played another, much more illustrious Time Lord off-screen: the Doctor himself in a new incarnation, first debuted in the play Doctor Who and the Daleks in Seven Keys to Doomsday and reprised in its audio drama adaptation Seven Keys to Doomsday.