Vislor Turlough

Turlough was posing as a public schoolboy at the school where the Brigadier was teaching A-level maths. Turlough was sent to kill the Doctor by the Black Guardian, but he couldn't do it and became one of the Doctor's companions. He was an alien stranded on Earth, and the Black Guardian told him that if he killed the Doctor, he would have his freedom. But, instead he decided to join the Doctor and he found himself helping the Doctor to beat the Master. Following the discovery of his younger brother Malkon on Sarn (Sarn is a volcanic world and a former Trion prison colony), Turlough finally revealed that he was from the planet Trion from which he had been exiled. He summoned a Trion spaceship to evacuate the population and his exile was revoked.

The Black Guardian is one of two forces to balance the universe. The Black Guardian wanted the Key to Time to cause havoc in the universe, but the Doctor got all of the missing pieces and destroyed the Key to Time. He then recruited Turlough and between the two Guardians, Turlough rejected the Black Guardian causing him to dissolve in flames, but he was not completely destroyed.

Turlough's home planet, Trion, is a technologically sophisticated civilization, who are governed by imperial clans. When civil war broke out, many people were exiled to Earth and other planets. Turlough, his brother and many descendants of Trion prisoners were eventually given their freedom and all charges against them were dropped. Turlough once said that his planet was invaded by the Tractators and when he and the Doctor met them, all of his memory of them came back. 

Vislor Turlough was in Mawdryn Undead to Planet of Fire, plus as a memory in The Caves of Androzani: Episode 4.