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
Attending Brendon Public School (which had amongst its staffmembers Brigadier Lethbridge-Stewart), Turlough was under the supervision of a Custodian masquerading as a solicitor in Chancery Lane. During the events of 'Mawdryn Undead', he was contacted by the Black Guardian who offered him his freedom in exchange for the death of the Doctor, and at the conclusion of that story he joined the Doctor with the aim of fulfilling his part of the bargain.

Over the Black Guardian trilogy, he became more and more loathe to complete this aim, until finally in Enlightenment he chose to destroy a prize that would have guaranteed his freedom and his pact with the Black Guardian rather than kill the Doctor, and became an active member of the TARDIS crew.

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" ('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.

Slippery and untrustworthy, at least from the outset, it was clear that Turlough's main focus was on his own personal survival. His character evolved and changed over the course of his adventures with the Doctor, leading to him deliberately putting his own life at risk to save the Doctor and Tegan ('Warriors of the Deep'), facing the demons of his own people ('Frontios') and finally accepting personal responsibility and trading in his own freedom to save the lives of the population of Sarn. ('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.

Continuity Error
Although through the entirity of the Black Guardian Trilogy 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.

Quotes
The King's Demons [To the Master] "I've had had quite enough of you, whoever you are - so don't try me too far!"

"What is it about Earth people that makes them think a futile gesture is a noble one?"

Singularity [When asked where his sense of adventure was] "Back in the TARDIS. In a box marked 'sense of adventure'"