Handles

"Handles" was the name given by the Eleventh Doctor to an empty Cyber-helmet that he had obtained from the Maldovarium Market, and subsequently repaired. All of its organic components and Cyberman protocols had been cleaned out, leaving it a plain, docile robot with a tendency to interpret commands literally (whether or not they were meant to be taken as rhetorical statements).

Handles became the only constant companion of the Doctor during his self-imposed exile on Trenzalore protecting the town of Christmas, during which period he eventually developed an irreparable fault and died, leaving the Doctor devastated.

Creation
One of several Cybermen on a mission to the the Maldovarium was beheaded by Kiz the Head-Taker during a fight after receiving the mocking nickname of "Handles". (PROSE: The Heist) With its organic contents and Cyber-protocols removed, the head in question, now answering to the name of Handles, became a harmless if odd-looking robot in its own right.

The Doctor, in his eleventh incarnation, purchased him while visiting the Maldovarium and brought him aboard the TARDIS, eventually figuring out that Handles could be plugged into the TARDIS control console and help him fly the ship, such as by operating its teleport system, allowing the Doctor could be beamed back inside his ship if he found himself in danger while out exploring. (TV: The Time of the Doctor)

Adventures with the Eleventh Doctor
Handles went on many adventures with the Eleventh Doctor as he searched for the mysterious planet of Trenzalore. When the Doctor went aboard ships orbiting Trenzalore before he gained access, Handles was accidentally brought aboard a cyber-ship, where the Cybermen attacked the Doctor.

Outliving the residents he had chosen to protect, Handles was the Doctor's only constant companion during his first three hundred years in the town of Christmas. (TV: The Time of the Doctor) On several occasions, the Doctor left Handles behind to look after the Clock Tower in Christmas. (PROSE: Let it Snow, An Apple a Day...)

He spoke with Handles about happiness after he had defeated the Krynoid during the harvest festival in Christmas. (PROSE: An Apple a Day...) He later spoke to Handles about the "Strangers in the Outland". (PROSE: Strangers in the Outland)

As the years passed, Handles began to corrode with rust and slowly lost his functionality. The Doctor kept repairing Handles as best he could, but he lacked enough spare parts and eventually broke down. Before shutting down, Handles finally fulfilled an order the Doctor had given him centuries earlier, prior to landing on Trenzalore: reminding the Doctor to patch the TARDIS phone back to the console. Handles' death reduced the Doctor to tears. (TV: The Time of the Doctor)

Legacy
In his delirious post-regenerative condition, the Twelfth Doctor briefly mistook Clara Oswald for Handles, despite the obvious physical differences between the two. (TV: Deep Breath) The Twelfth Doctor later referred to Handles as a "great companion." (PROSE: The Mondas Touch)

Abilities
Handles still carried a full set of databanks from his time as part of a Cyberman. The Eleventh Doctor used Handles to access and compute that data. Handles was also a source of advice and conversational partner to the Doctor, with varying degrees of success due to his literal computer brain. However, despite his apparent skulduggery, Handles was not without a mind of his own, to the point that the Doctor once referred to Handles "dreaming" (TV: The Time of the Doctor).

Handles was able to interface with the Doctor's TARDIS itself, piloting the ship as well as making use of its databanks and other systems, via a custom post on the console he could be mounted onto. When the Doctor was wearing a communicator earpiece, Handles could maintain contact with him from within the TARDIS and teleport him aboard when needed. Handles could also home the TARDIS in on the Doctor and materialise it around him when the Doctor activated his TARDIS key.

The Doctor occasionally brought Handles along on trips outside the ship. When he was being shot at by Cyberman, the Doctor exploited a Cyberman suit's resistance against its own weaponry and used Handles to block blasts from other Cybermen's wrist blasters. (TV: The Time of the Doctor)

Behind the scenes

 * Discounting the unique case of the Doctor's TARDIS, Handles is the longest-serving companion, in terms of time, the Doctor is known to have had, as he was with him for the first 300 years he was on Trenzalore. Clara Oswald was the Doctor's "current" companion for the entire 900 years he was on Trenzalore, but she wasn't with him continually, and for her, only a few hours passed.
 * Handles is the second Cyberman companion of the Doctor, the first being Kroton. However, unlike Kroton, Handles was only made of Cybertechnology (he was purely electronic and just a head), while Kroton was a genuine Cyberman, a human cyber-conversion victim. However, both of them share the distinction of being separated from their intended Cyberman protocols.
 * Handles is the only on-screen companion in Doctor Who to be considered a Gadget, not a playable character, by the Doctor Who Roleplaying Game.
 * Handles was supposed to appear as a hallucination when the Eleventh Doctor regenerated, along with other of the Doctor's companions. In the end, only Amy appeared.