Fey Truscott-Sade

Fey Truscott-Sade was an undercover agent for the British government in 1939, and saved the Doctor by managing to pilot the TARDIS back to Gallifrey, where he was healed of a rare virus. The Doctor was suspicious of this, however, and it emerged that Fey was under the control of a secret Time Lord sect called the Elysians. Shayde, an amalgm creature created by the Gallifreyan Matrix and an old friend of the Doctor, foiled this attempt and made a plan with the Doctor to expose the Elysians.

Throughout the Doctor's following adventures, this plan came into use, and it is discovered that Shayde must fight against the Pariah, an immensly powerful version of himself. Shayde lost, and was about to die when Fey invited him to merge with her body, so giving Fey Shayde's powers. She then left the Doctor and Izzy Sinclair, calling herself Feyde, as she and Shayde needed to get used to their new arrangement.