The Doctor (From Eternity)

This incarnation of the Doctor worked on behalf of the Time Lords. On one occasion, he interrogated an entity that destroyed countless lives because it perceived time backwards and tried to save the the species from devolving. (ST: From Eternity)


 * It is possible that this was a younger First Doctor before his exile, the Second Doctor working for the Celestial Intervention Agency, or a future Doctor who had returned to Gallifrey (possibly the Eighth Doctor on the reconstructed Gallifrey after The Gallifrey Chronicles).