Daxtar

Daxtar was the manager of a research station on Earth in the year 4000 and a friend of Bret Vyon.

Bret came to him with the First Doctor and Steven Taylor, believing that he could help them fight against the Daleks and Mavic Chen. However, Chen had already convinced Daxtar they were traitors and had informed him about the taranium core. Daxtar gave himself away when he mentioned the taranium, which he otherwise could not have possibly known about.

When the Doctor pointed this out, Daxtar tried to defend himself by telling Bret the Doctor had mentioned it, but Bret, angry at his betrayal, shot and killed him. (TV: The Daleks' Master Plan)