Daxtar

Daxtar was the manager of a research station and a friend of Bret Vyon in 4000. Bret came to him believing that Daxtar could help them fight against the Daleks and Mavic Chen, but Chen had gotten to him first. Daxtar gave himself away when he mentioned the taranium core, which he otherwise could not have possibly known about. When the First Doctor pointed this out, Bret killed him for his betrayal. (TV: "The Traitors")