Handles

"Handles" was the name given by an Auton copy of Dorium Maldovar to a severed weapons-grade Cyberman head during a theft of the Maldovarium by Kiz the Head-Taker, Vertebrae and Agrakos. (PROSE: The Heist) The Eleventh Doctor eventually obtained Handles from the Maldovarium and subsequently repaired it. All of its organic components and Cyberman protocols had been erased, 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. (TV: The Time of the Doctor)

The Maldovarium
"Handles" was originally one of twelve Cybermen, formerly of the Cyber-Legions, whom Dorium Maldovar had acquired and modified to function as a patrol of guards for the Maldovarium. When Kiz the Head-Taker, Vertebrae and Agrakos attempted a theft of the Maldovarium, Handles, along with all the other Cybermen, was beheaded by Kiz, before being used to keep a door wedged open for when the three criminals escaped, during which an Auton copy of Dorium gave Handles its nickname. Later, when Kiz chose to escape alone, he found that somebody had moved Handles from the door, cleaned it, and placed the Cyberman head in the centre of the door's service duct, locking him and the others inside the Maldovarium. (PROSE: The Heist)

Adventures with the Eleventh Doctor
The Eleventh Doctor eventually purchased Handles - who by this point had been stripped of all Cyber-protocols and organics - 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 to be beamed back inside his ship if he found himself in danger while out exploring.

When a message began repeating throughout time and space, the Doctor followed it to its source, the planet Trenzalore, discovering a fleet of parked starships. Handles teleported the Doctor aboard several of the ships only to teleport him back when the crews inevitably opened fire on him. Handles was later brought aboard one of the ships as a symbol of comradeship only for the Doctor to discover it was a cyber-ship, the Cybermen opening fire with the Doctor using Handles' immunity to cyber-weapons as a shield. Following Clara Oswald joining them, Handles' analysis of the message led to him identifying the planet as Gallifrey, finding the message to be of Gallifreyan origin. When the Doctor and Clara investigated the message in person, the Doctor affixed a Seal of the High Council to Handles allowing him to fully decode the message which Handles broadcast to the entirety of the orbiting fleet: "Doctor Who?". When the Doctor tricked Clara home as he mounted a defence of the planet, Handles remained with the Time Lord.

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...)

The Doctor spoke with Handles about happiness after he had defeated a 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) On Catrigan Nova, this Doctor later referred to Handles as a "great companion." (PROSE: The Mondas Touch)

Historians from an era that succeeded the Siege of Trenzalore recorded that the Doctor had used "Cyber-Technology" to decode the Question during the Siege's opening stages. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

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".

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 TARDIS. When he was being shot at by Cybermen, 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 in-universe 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.