Vislor Turlough — more commonly just "Turlough" — was a companion of the Fifth Doctor who principally traveled alongside Tegan Jovanka, but also had a few adventures with Nyssa and Kamelion. He also briefly met and saved the life of Peri Brown before departing the Doctor's company.
Turlough was a complex individual, whose motivations were often unclear to those around him. He was for a time — by his own choice — under the influence of the Black Guardian. During this period he was actively trying to kill the Doctor. Later, after overcoming the Black Guardian, he was mostly a trusted member of the TARDIS crew, but there remained instances where his actions were questioned. It was only at the end of his time on the TARDIS that the Doctor finally discovered the truth about how Turlough ended up on Earth, and why he was susceptible to the Black Guardian in the first place.
Turlough acted selfishly, always preventing himself from blame. He could lie convincingly to most though some, such as Tegan and the Brigadier, were not taken in by him.
Tegan did not like Turlough, finding him untrustworthy. She did not approve of the Doctor letting Turlough consider the Black Guardian's offer, believing that Turlough would choose the crystal. However, the Doctor was confident in Turlough's morals.
Despite this, he recognised the Doctor's good intentions; he tried to make the Black Guardian reconsider his order to kill the Time Lord. He understood the TARDIS systems well enough to be able to run a diagnostic in The Five Doctors and programmed the TARDIS to rescue the Doctor and Peri in Planet of Fire.
Turlough was the last male companion to travel with the Doctor on-screen until Adam Mitchell, who joined in Dalek in 2005