Mavic Chen

Mavic Chen was the Guardian of the Solar System around the turn of the 41st century, entrusted with the security of Earth and the neighboring planets. However, Chen was corrupted by a lust for power, and gladly betrayed his own people, allying himself with the Daleks and giving them the taranium they needed to complete their Time Destructor.

When the Doctor and his companions succeeded in stealing the taranium from the Daleks' base on Kembel, Chen at first recruited Space Security Agent Sara Kingdom to help him, falsely telling her that her brother Bret Vyon, who was helping the Doctor, was a traitor. Sara believed Chen and killed her brother, but quickly switched over to the Doctor's side upon learning the truth.

When the Doctor attempted to elude his enemies in the TARDIS, Chen joined the Daleks in their pursuit of the time travellers through the centuries. Chen was eventually able to retrieve the taranium after the Daleks caught up with the Doctor in ancient Egypt. Chen and his allies returned to Kembel, where he was finally exterminated by the Daleks, having served out his usefulness. By now insane, Chen proclaimed he was immortal even as the Daleks' disruptors fired upon him. The Daleks soon followed him in death, as the Doctor activated the Time Destructor himself and destroyed all life on Kembel.

The Expanded Universe
According to the New and Missing Adventure novels, despite Chen's betrayal of Earth and his extermination by the Daleks, he succeeded in founding a dynasty that controlled Earth and much of the galaxy for many centuries.