Turlough was a complex individual. His motivations were often unclear to those around him. He was for a time willingly under the influence of the Black Guardian. During this period, he tried to kill the Doctor. After overcoming the Black Guardian, he was a mostly-trusted member of the TARDIS crew, but there were still moments 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.