Bret Vyon

Bret Vyon was a Space Security Agent who assisted the Doctor in his quest to stop the Daleks from completing their Time Destructor and using it to conquer Earth. Some authorities list Vyon among the Doctor's companions, while others do not. When he met the Doctor, Vyon had been on Kembel with fellow agent Kurt Gantry to investigate suspicious activity on the part of Mavic Chen, Guardian of the Solar System.

Once Vyon learned that Chen had allied himself with the Daleks, he stole Chen's spar with the Doctor, Steven, and Katarina in tow, hoping to reach Earth in time to warn its government. Chen, however, had convinced Bret's sister Sara Kingdom, also a Space Security Agent, that her brother was in fact the traitor, and acting on this belief Sara killed him. Once she learned the truth, Sara quickly allied herself with the Doctor, both to save Earth and avenge her brother's death.