Guy de Carnac was a member of the Knights Templar. He left the order in disgrace and became a wandering mercenary.
He had at least one run-in with the Inquisition. (AUDIO: The Quality of Mercy) The Seventh Doctor and Benny met him in 13th century France when they used the Jade Pagoda to escape a black star. Benny fell in love with Guy, but the romance ended tragically, as he was killed saving her and the Doctor. (PROSE: Sanctuary)
The Doctor created the John Smith persona to help him understand the human emotion of grief that Benny was feeling after Guy's death. (PROSE: Human Nature) Years later, when Benny had a baby, she named him Peter Guy Summerfield, to honour Guy. (PROSE: The Glass Prison)