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 Tutur for the Doctor at the Prydonian Academy. The Doctor would later consider him as the best one he ever had. (TV: The Twin Dilemma)

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 regenerations.

Recently regenerated himself, the Sixth Doctor, along with Peri and Hugo Lang, came to Jaconda in search of the twins. 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. When Mestor took possession of Azmael's body and the Doctor destroyed Mestor's real body, Azmael, with Mestor's spirit within him, forced himself to die by inducing a last regeneration. This killed both Azmael and Mestor. (TV: The Twin Dilemma)