Fey Truscott-Sade

Fey Truscott-Sade was an undercover agent for the British government, later a companion of the Eighth Doctor. She merged with Shayde into a being known as Feyde.

Biography
On an unknown date, Fey encountered the Doctor (presumably in his eighth incarnation) in an adventure involving psychic weasels in Russell Square. On that occasion the Doctor gave her a Stattenheim Summoner (a device that could contact the Doctor's TARDIS, disguised as a tin whistle). In 1939, Fey called on the Doctor and his companion Izzy to help her investigate the sinister Varney. (DWM: Tooth and Claw)

Unknown to the Doctor, in 1937, time travelling enemies of the Doctor's called the Threshold had kidnapped Fey and put a mind control device in her head. (DWM: Wormwood)

The defeat of Varney proved costly for the Doctor. He had injected himself with a deadly bacillus and was dying. (DWM: Tooth and Claw) Now secretly under the control of the the Threshold (DWM: Wormwood), Fey piloted the TARDIS to Gallifrey, where the Doctor's body was cured and his mind placed into the Matrix to heal. (DWM: The Final Chapter)

In the Matrix, an attempt was made on the Doctor's life by a secret Time Lord sect called the Elysians. The plot was foiled by Shayde, an artificial being created on Gallifrey. The assassination attempt was part of a plot by Overseer Luther. The Doctor thwarted Luther by short-circuiting his watchtower (a gigantic TARDIS), but apparently at the cost of his eighth body. As Izzy and Fey watched, the Doctor regenerated into a new incarnation. (DW: The Final Chapter)

However, this was a ruse. The Doctor had realised Fey was under external control when she had piloted the TARDIS even though the TARDIS manual was in Gallifreyan script, which she couldn't have understood. Just before the Doctor prepared to sacrifice himself, Shayde offered to take his place and fake a regeneration. This would give them the opportunity to bring a newly regenerated and weaker Doctor to them. Both disguised with personal chameleon circuits, Shayde would hold their attention while the Doctor sabotaged the Threshold's operations.

In Wormwood, a Wild West town on an alien moon run by the Threshold, the Doctor's plans came to fruition. The Threshold removed their implant from Fey, gloating she had been an unwitting spy for them. Preparing for the final confrontation, Shayde and the Doctor revealed themselves to the Threshold. However, Shayde had to contend with the Pariah, an immensely powerful being who was a prototype version of himself. Shayde could not defeat the Pariah on his own, and she crushed his skull. Comforting the dying Shayde, Fey merged with him and together they killed the Pariah and eliminated the Threshold. The shared being was dubbed Feyde by the Doctor, although both of them retained their own consciousness, decided to leave and deal with what had just happened to them. (DWM: Wormwood)

Fey returned to 1939, where she continued to work for the British government. Fey spent two years fighting the Nazis with Shayde, though she felt some frustration because Shayde would not allow her to use their powers to kill Adolf Hitler, as this would have changed history. Over the course of their union, Fey and Shayde began to develop a close love/hate relationship, sharing secrets, but often vigourously disputing the use of their power to influence history. Fey resented Shayde's unwavering loyalty to Rassilon and his lack of individuality. These traits implied she was in love with Shayde, but could not admit this to herself or her partner.(DWM: Me and My Shadow) In 1941, she received a sub-ether summons from the Doctor — Izzy had been kidnapped and the Doctor needed Shayde's abilities to track her on the planet Oblivion. They recovered Izzy. After Izzy was restored to her own body after having exchanged it with Destrii's, Fey encouraged her (Izzy) to embrace her true sexuality and gave her a passionate kiss. Feyde returned to World War II. (DWM: Oblivion)

Powers
As a merged entity, Fey was able to use Shayde's powers, including the ability to travel through time and space unaided, phase through solid objects and fire self-generated "psychic bullets".

Sexuality
Fey's penchant for dressing in masculine clothing and her relationship with Izzy (herself a lesbian) implied that Fey was a lesbian, but her actual orientation was never made explicit. However, given certain dialogue with the Doctor, there is evidence that she could be more accurately described as bisexual.