Vislor Turlough

Probably best known as the only companion who deliberately tried to kill The Doctor of his own free will, Turlough was a political prisoner, exiled to Earth after a civil war on his home planet of Trion. Desperate to escape, he agreed to an offer made to him by the Black Guardian - kill the Doctor and regain his freedom.

Biography
Little is known about Turlough's life on Trion before and during the civil war, although his desperation to destroy a distress beacon he recognises as being Trion in origin, and his reluctance to contact his own people to save the people of Sarn indicate that there were severe penalties for him escaping his exile. It can be reasonably assumed he was an active member of the military, as he was assigned the rank and serial number of "Junior Ensign Commander Vislor Turlough. VTEC9/12/44" (DW: Planet of Fire) His mother was killed during the civil war, and his younger brother Malkon and father exiled to the abandoned colony planet of Sarn, whilst Turlough himself was exiled to 20th Century Earth. (DW: Mawdryn Undead)

On Earth he lived as a human schoolboy, hiding his alien origins and attending Brendon Public School (which had amongst its staffmembers Alistair Gordon Lethbridge-Stewart), Turlough was under the supervision of a Custodian masquerading as a solicitor in Chancery Lane. He was contacted by the Black Guardian, who offered him his freedom in exchange for the death of the Doctor. Turlough subsequently ingratiated himself with the Doctor as his companions Nyssa and Tegan Jovanka, assisting in resolving a time paradox, after which he was invited to join the Doctor in his travels (DW: Mawdryn Undead).

In subsequent adventures, Turlough received recurring messages from the Black Guardian, reminding him of his obligation to kill the Doctor, an act he became more loath to do as he grew fonder of the Time Lord. Finally, he chose to destroy a prize that would have guaranteed his freedom and his pact with the Black Guardian rather than kill the Doctor, ending his association with the villain and becoming a loyal member of the Doctor's team. (DW: Enlightenment)

It was after breaking away from the Black Guardian that Turlough's outlook changed from one of self-preservation to wanting to help others, leading to him deliberately putting his own life at risk to save the Doctor and Tegan (DW: Warriors of the Deep), facing the demons of his own people (DW: Frontios) and finally accepting personal responsibility and trading in his own freedom to save the lives of the population of Sarn. (DW: Planet of Fire)

After discovering his younger brother was still alive, and the persecution of political prisoners had long been abandoned on Trion, Turlough left the TARDIS to return to his home planet, after effectively putting the Doctor ("who gets into the most terrible trouble") in the hands of new companion Peri Brown (DW: Planet of Fire). Later, as he regenerated, the Doctor had visions of several of his past companions, including Turlough (DW: The Caves of Androzani).

Not much is known of Turlough's life after leaving the Doctor, although it is known that soon after his return to Trion he became involved in an adventure to save both his homeworld and the planet Earth (Turlough and the Earthlink Dilemma).

There is also a possibly apocryphal account that suggests Turlough might have briefly travelled with the Sixth Doctor and Peri (Crisis in Space).

Behind the scenes
Turlough is the last companion of the classic series to travel with the Doctor on-screen who is not of the human race as well as the last male companion up till Adam Mitchell who joins at the the episode Dalek in 2005.

Also, Turlough is currently the last to be an alien companion. (Astrid never did travel with the Doctor and she is from the Planet Sto)

Continuity Error
Although through the entirety of the 'Black Guardian Trilogy' (DW: Mawdryn Undead, Terminus and Enlightenment) Turlough is desperate to return to his home planet, and indeed, at the end of Enlightenment requests that the Doctor take him home, by Planet of Fire he is actively trying to avoid returning home, destroying a distress beacon from a Trion ship, attempting to disable Kamelion, whom he believed was reacting to the beacon, and showing extreme reluctance to contact his own people -- knowing full well that it would mean the end of his freedom.