Azmael

Azmael was a renegade Time Lord who emigrated from Gallifrey to the planet Jaconda, where he ruled compassionately until Mestor deposed him.

Early life
Azmael was a tutor for the Doctor at the Prydonian Academy. The Doctor would later consider him as the best one he ever had. (TV: The Twin Dilemma) Azmael was present when the Doctor was expelled from the Academy. (PROSE: Divided Loyalties)

At some point during the Doctor's fourth incarnation, the Doctor and Azmael had an encounter in which Azmael drank so much the Doctor had to throw him into a fountain to sober him up. (TV: The Twin Dilemma)

Thirteenth Azmael
When the villainous Mestor, who commanded the Gastropods, came to power on Jaconda, he used a telepathic link with Azmael to dominate his mind. Mestor forced Azmael, who used the name Professor Edgeworth, to kidnap the two young genius identical twins Romulus and Remus Sylvest. By this time, Azmael had reached the end of his regeneration cycle.

Recently regenerated himself, the Sixth Doctor and Peri came to Titan III. There they saw the wreckage of a ship and rescued Lieutenant Hugo Lang. While exploring some tunnels on the asteroid, Peri and the Doctor were captured and taken to meet Azmael. The Doctor recognised Azmael as a friend from their homeworld of Gallifrey. Co-operating with Mestor to save his people, Azmael abandoned the Doctor and Peri, letting them be taken captive. However, when the Doctor and Peri were able to follow him to Jaconda, Azmael sided with the Doctor when his observations revealed Mestor's true plan to trigger a solar eruption that would destroy Jaconda and send Mestor's eggs out across the universe. When Mestor took possession of Azmael's body and the Doctor destroyed Mestor's real body, Azmael, with Mestor's consciousness within him, forced himself to die by attempting to regenerate. Since Amzael had already used all twelve of his regenerations, this attempt at a thirteenth regeneration resulted in both his and Mestor's deaths. (TV: The Twin Dilemma)