The Keeper of Traken, or simply the Keeper, was the leader of the planet Traken and of the Traken Union. The Keeper had the power to control everything in the Union, making sure it was harmonious. (TV: The Keeper of Traken)
The Keepers ruled for thousands of years with the power of the Source. The Source allowed Keepers to take control of minds and to materialise anywhere. Drawing on all the minds of the Union, they protected the planets. Every thousand years, a new Keeper would be chosen when the old one died.
One Keeper asked the Fourth Doctor for assistance in preventing the takeover of the bioelectronic Source by evil. After this Keeper's death, Kassia briefly took over the role, before she was killed and replaced by the Master. After the Master was stopped by the Doctor, Luvic became the Keeper. (TV: The Keeper of Traken)