John Wilkes is an Australian who believes that Sauniere discovered lost treasure in a cave hidden somewhere beneath the Rennes-le-Château area. He carries a seismic survey machine that becomes the butt of ridicule for Gabriel. Wilkes himself appears arrogant at times and constantly tries to hit on Madeline, with little success. He later turns his interests towards Grace, who plays along in order to discover Wilkes' findings. He has a musculine built and a prominent moustache.
He, in fact, discovers the temple and its entrance and is killed by Montreaux's lackeys as a consequence. His discoveries do help Grace in her investigation.