Shayde

Shayde was an instrument of the Matrix Lords that was placed on the TARDIS without the Doctor's knowledge to represent Gallifreyan interests. In time, the Doctor's fifth and eighth incarnations found a way to work with Shayde.

As a construct rather than a fully organic being, Shayde was able to both emulate and merge with other beings. During his adventures with the Doctor, Shayde joined with Fey Truscott-Sade to become "Feyde".

As Shayde
The minds of physically dead Time Lords in the Matrix created Shayde as a successor to the Pariah. (COMIC: Wormwood) Like all casts, (PROSE: The Book of the War) he was used as an instrument by the Matrix Lords in the greater universe. (COMIC: The Tides of Time) By another account, Aspect Black was originally a Time Sentinel who acted as a lone agent. (COMIC: The Good Companion)

Shayde first met the Doctor when he helped the Fifth Doctor defeat Melanicus. At first he only acted covertly, but he physically manifested himself after the Time Lords increased his power levels. (COMIC: The Tides of Time)

He aided the the Fifth Doctor against another force, a mutated Gallifreyan creature called a rovie that had infiltrated the Doctor's TARDIS and was trying to kill the Doctor and his companions Peri and Erimem. (AUDIO: No Place Like Home)

Shayde then saved the Eighth Doctor from an assassination attempt in the Matrix by the Elysians. This was part of a plot by Overseer Luther. Before the Doctor sacrificed himself to defeat Luther, (COMIC: The Final Chapter) Shayde took his place and faked a regeneration as bait for the Threshold, who would eagerly take the opportunity to attack a weaker, newly-regenerated Doctor. (COMIC: Wormwood)

In the appearance of a new incarnation of the Doctor, he took the Doctor's companions Izzy and Fey to the Threshold stronghold of Wormwood. Shayde and the Doctor, who was disguised with a personal chameleon circuit, eventually revealed themselves to the Threshold, and Shayde tried to defeat the Pariah. However, he failed, and she crushed the globe that formed his head. However, Fey merged with the dying Shayde.

As Feyde
United into one, Fey and Shayde successfully killed the Pariah and defeated the Threshold. Though they were a single being, which the Doctor dubbed "Feyde", they each retained their own consciousnesses, and they decided to leave the Doctor to deal with what had just happened to them. (COMIC: Wormwood)

They returned to Fey's original time period, the 1940s, where as an agent of the British government they spent two years fighting the Nazis. Shayde refused to allow Fey to use their powers to kill Adolf Hitler and change history, causing the two to develop a complex relationship. In 1941, they received a sub-ether summons from the Eighth Doctor, saying that Izzy had been kidnapped and the Doctor needed Shayde's abilities to track her whereabouts. (COMIC: Me and My Shadow) Feyde succeeded in recovering Izzy, and they returned to World War II. (COMIC: Oblivion)

Feyde would later become an important ally of the War Doctor in the Last Great Time War. Fey was seriously injured during a battle against the Morlontoa, but they teleported to the college dormitory of her future nephew Alexander Truscott. As Fey healed, she subconsciously separated Shayde's mind from hers, hiding his essence in the form of all CCTV cameras in London. However, she believed she was working under Truscott, who manipulated her into turning against the Time Lords and then used Shayde's powers to create the Clockwise Men. Shayde finally reconstituted himself in the form of a giant black sphere around Fey as Truscott's plan started. This sphere allowed the Twelfth Doctor and his allies to travel to the Dreamspace where they were finally able to teach Fey the truth of her being the cause behind the battling.

With Fey's illusion broken, the black sphere shattered as well, and in the real world, Shayde reappeared in his normal form, dying. However, he transferred the last of his essence to Fey to keep her alive, while also wiping her memories of her time bonded with him. Relieved that he had finally put Fey's mind at rest, Shayde disintegrated, much to the Doctor's sadness. (COMIC: The Clockwise War)

Powers
Shayde had many powers, (COMIC: The Tides of Time, et. al.) including the abilities available to all casts: (PROSE: The Book of the War) He could also track individuals through time and space. (COMIC: Oblivion)

In his original form, he could also remove his spherical "head" and store things in his "body" via the resulting dimensionally-transcendental gap. (AUDIO: No Place Like Home)

Behind the scenes

 * Shayde's origin was given contradictory descriptions in the stories The Good Companion and The Book of the War: in the former, he is a Time Sentinel; in the latter, he is a cast.