Vislor Turlough (Mark Strickson)
Turlough was at Brendon School, in England, when he was approached by the Black Guardian, who persuaded him to help destroy the Doctor. Turlough's allegiances were tested when the Doctor saved Turlough's life.
Gradually it was revealed that Turlough was an alien, marooned on Earth after a civil war on his own planet, Trion, which had claimed the life of his mother. His father and brother had been sent to the prison planet Sarn, where Turlough eventually decided to leave the Doctor.
His time on board the TARDIS had been a difficult one, and he was never entirely trusted by his travelling companions.