Mr Qixotl was the pseudonym of a humanoid of undetermined species. Qixotl owned a TARDIS, though he no longer considered it to be one since he had modified it extensively to prevent the Time Lords from tracking him down.
Qixotl betrayed the Doctor at least a couple of times before the outbreak of the War: on one occasion Qixotl turned the Doctor over to an “Embodiment of Pure and Irredeemable Evil”; another incident – which occurred for the Doctor two regenerations after the previous one – had Qixotl trying to sell the Doctor to Antiridean organ-eaters. Qixotl had to change his face after the Doctor did something to him.
Qixotl held an auction on Earth in the late 21st century. The sole item for sale was the Relic, whose existence he learned of while stranded on Dronid. During the course of the auction, Qixotl was killed by the Krotons, but recorporated by the Celestis in exchange for the Relic. (Alien Bodies)