Quoted from Wikipedia:
The Stig is a character in the British motoring television show Top Gear. The character plays on the anonymity of racing drivers’ full-face helmets, with the running joke that nobody knows who, or indeed what, is inside the character’s racing suit.
The Stig is responsible for setting lap times for cars tested on the show, as well as instructing celebrity guests in the show’s Star in a Reasonably Priced Car section. Several episodes have featured comic variations on the Stig character, referred to by the presenters as ‘The Stig’s cousin’. The Stig character originally wore a black helmet and overalls, but he was killed off in the first episode of series three, to be replaced in the next episode by a new Stig, wearing white helmet and overalls. The BBC have always refused to officially confirm the identity of the actor who plays the Stig. “Who is The Stig?” became a frequently asked question on the internet.