The Gravis

The Gravis was leader of the Tractators who inhabited the planet Frontios. The Gravis was the only sapient Tractator. He controlled the others mentally.

The Gravis appeared to confide in the Fifth Doctor, viewing him more favourably than the human colonists on Frontios. The Gravis was more powerful and more intelligent than other Tractators. He was tricked by the Doctor into reassembling the Doctor's TARDIS. However, the effort weakened him severely. The Doctor took him away from Frontios in the TARDIS. (TV: Frontios)

After leaving Frontios, the TARDIS was diverted to Queth, where Gravis helped the Doctor and his companions in their efforts to save the planet from destruction. Afterwards, the Doctor left him on an uninhabited, rocky planet called Kolkokron. (PROSE: Life After Queth)

He later helped the Sixth Doctor to stop Stream's plan to create a gravity machine. (AUDIO: The Hollows of Time)