Stefan was a human from 12th century Earth. While in Constantinople with Barbarossa during the Third Crusade, he played a game of dice with the Celestial Toymaker. He lost, becoming the Toymaker's first servant.
Stefan was sadistic, often enjoying carrying out the Toymaker's more cruel instructions. While the Sixth Doctor was playing the Great Game with the Toymaker in 1986 Blackpool, the Doctor was able to overpower Stefan and throw him in the path of an attacking monster in the Game, which killed Stefan. (PROSE: The Nightmare Fair, AUDIO: The Nightmare Fair)