Nardole

Nardole was a companion of the Twelfth Doctor and former employee of River Song.

While working for Dr Song on Mendorax Dellora, he was beheaded by the body of King Hydroflax, who used him to obtain information on her whereabouts. (TV: The Husbands of River Song)

Knowing River would go to her death in the Library following their twenty-four years together on Darillium, the Doctor reassembled Nardole out of fear of being alone. (TV: The Return of Doctor Mysterio)

River's final instruction for Nardole was to save the Doctor from executing. He did so, but not before the Doctor swore an oath to protect her body for a thousand years. Missy was placed in a Quantum Fold Chamber and Nardole kept the Doctor to his word. (TV: Extremis)

Life before the Doctor
Nardole had an ex called Velma, who worked as an actress. She left him for an AI at a call centre. He once swapped his face while "on the run". (TV: Oxygen)

Working for River Song
Nardole first encountered the Doctor while working for River Song on Mendorax Dellora in 5343. He was told to find the surgeon. The surgeon was required as River Song needed to extract the Halassi Androvar from the head of King Hydroflax. However, Nardole accidentally found the Twelfth Doctor and brought him to Professor Song instead.

When River and the Doctor eventually escaped Hydroflax's ship with his head in a bag, Hydroflax's mechanical body turned to Nardole and wanted to extract strategic information. Although Nardole claimed to know very little of River's plans, there was only one way for Hydroflax's body to be sure, so it removed Nardole's head and attached it to its own. Later, whilst holding a gun to Nardole's head, Hydroflax's body forced him to draw in Ramone, whose head the body took while storing Nardole's within itself.

On the Harmony and Redemption, the body pulled Nardole's head out of storage to confirm River's identity before putting it back against his protests.

After the crash of the Harmony and Redemption on Darillium, Hydroflax's body was recovered from the wreckage. As the hostile onboard computer was destroyed by the Doctor, Nardole and Ramone shared control of the body, working in a restaurant situated before the Singing Towers. When River arrived several years after the crash, Ramone was in control as Nardole was getting some "me time" inside the body, something he admitted was hard when he greeted River from inside Hydroflax's body. (TV: The Husbands of River Song)

24 years on Darillium
The Twelfth Doctor and River Song spent their final 24 years together on Darillium. During this time, the Doctor grew fearful being alone, and so returned to Nardole and reassembled him, taking his head from Hydroflax and "glueing" it back on. (TV: The Return of Doctor Mysterio)

At River Song's final request, Nardole followed the Doctor to a planet where was to be executed. Here, the Doctor swore an oath to protect her body for a thousand years. Disguised as a priest, Nardole intervened in the proceedings and instructed him not to kill Missy as his late wife wouldn't approve. The Doctor let Missy live, and he and Nardole locked her in a Quantum Fold Chamber. At the Doctor's oath, the pair would guard the vault for next thousand years. (TV: Extremis)

Guarding the Quantum Fold Chamber
Nardole became the Doctor's companion, and saw it as his duty to ensure the Doctor was all right following River's death. (TV: The Return of Doctor Mysterio)

While guarding the vault, (TV: Extremis) the pair still found time for an Earth-based adventure. (TV: The Return of Doctor Mysterio) Together they investigated Harmony Shoal with the help of "The Ghost". He also spent some time as the Emperor of Constantinople, in which he ruled "firmly, but wisely". (TV: The Return of Doctor Mysterio)

The Doctor and Nardole settled at St Luke's University in Bristol, where they spent nearly seventy years watching over the vault which was now hidden beneath the campus. While stuck on Earth, the Doctor took a job as a lecturer and Nardole acted as his valet. It was there where the Doctor, canteen worker Bill Potts and Nardole encountered a sentient oil puddle. Realising it was after them and not the vault, the Doctor, Bill and Nardole tried to evade the creature, but it followed them through time and space. They travelled to a war zone in the Dalek-Movellan War, where Nardole ran interference and sealed off the area of all but one of the Daleks. The last Dalek was actually the oil creature, which had taken over the body of Heather, and remembered Heather's promise to never leave Bill. Bill released "Heather" of her promise, and the trio returned to St Luke's. (TV: The Pilot)

When the Doctor formally invited Bill to join him as his companion, he sent Nardole to put the kettle on in the Doctor's office whilst he secretly introduced her to all of time and space. (TV: Smile) Bringing tea with a "bit of coffee" in it to "give it some flavour", Nardole was dismayed to learn that the Doctor and Bill had been to Regency London while he was out, which he considered "naughty" of the Doctor as he had broken his oath, only for the Doctor to argue that his return proved he kept it. After being told by the Doctor to "leave [him] alone", a frustrated Nardole voiced his frustrations to himself while standing guard outside the chamber, before being interrupted as Missy started heavily knocking on the door. Though acknowledging that the Doctor had been "distracted" by Bill, Nardole, hiding his fear, asserted to Missy that he would still stand guard and would not let her escape. (TV: Thin Ice)

After the Doctor had saved Bill and her friends from a creaking house and a creepy Landlord, the Doctor gave Nardole the night off, much to his annoyance. He was disappointed when he learned that the Doctor had put a piano in the Vault, saying that he "never learns". (TV: Knock Knock)

After introducing Bill to space-time travel, the Doctor often left Nardole on Earth whilst the two had adventures. (PROSE: The Shining Man, Diamond Dogs)

Travels with the Doctor and Bill
Going to desperate measures to ensure that the Doctor kept his oath and remained on Earth guarding the vault, Nardole removed Fluid link from the TARDIS after having been informed by the Doctor that the TARDIS couldn't travel without it. The Doctor revealed to both Nardole and Bill that he'd lied and took them on an adventure to Chasm Forge, a space station from which the TARDIS had detected a distress signal.

On the space station, the trio were met with dangerous consequences where oxygen was lacking and needed paying for and the crew's spacesuits wanted to kill them. Finally, the three returned to Earth and Nardole revealed his relief that the Doctor hadn't been injured or killed on the adventure as the person within the vault would be able to sense his weakness. However, the Doctor revealed that he'd been blinded by being exposed to the vacuum of space for too long. (TV: Oxygen)

Soon after being informed by the Doctor, who'd received a message from his virtual self, about an alien race - who had been using a simulator of Earth and all it's inhabitants; including Nardole himself, as means to discover the plant's weaknesses - Nardole accompanied his friends to Turmezistan where a 5,000 year old pyramid stood which hadn't been there the day before.

It was not long before it was revealed that the Monks were behind the pyramid's erection and, after displaying their powerful abilities of teleportation and telekinesis, the Monks revealed that they wished to rule the Earth but would only do so when they were asked as ruling the population would be easier if they went along with the Monk's plan. (TV: The Pyramid at the End of the World) After a time, Nardole joined the Doctor and Bill on off-world adventures. The trio visited Edinburgh in 1645. (PROSE: Plague City)

Personality
Nardole was loyal and humble. He saw it as his duty to make sure the Doctor was alright following the death of River Song. (TV: The Return of Doctor Mysterio) However, Nardole was far from submissive. He took his position with a firm attitude, strongly insisting the Doctor stay true to his oath that prevented him from going off-world outside of emergencies. The Doctor even referred to Nardole as his "mum" because of this. (TV: Smile) Nevertheless, he was also rather timid and could show great fear when faced with danger. He was particularly scared of heights. (TV: The Husbands of River Song)

He was easily deceived. He mistook the Doctor for "the best surgeon in the galaxy". (TV: The Husbands of River Song) and when asked to fetch a packet of crisps from Birmingham to Bristol, he described it as "a cunning ruse". (TV: Oxygen) Although he could sometimes appear incompetent, he was also knowledgeable and, despite not originating from Earth, knew that New York wasn't a capital city, unlike Washington DC. (TV: The Return of Doctor Mysterio)

Nardole could also be child-like. He happily played with children's toys (TV: The Return of Doctor Mysterio) and enjoyed a "cuddle" with the Doctor and Bill. (TV: Oxygen)

Nardole appeared to have a distaste for humans. He refused to make Bill tea because he's "not a slave for any human, that's for sure". (TV: Smile)

Nardole has suggested a possible criminal past. He referred to a time when he was "on the run", and was quick to deny that he was capable of both murdering and robbing the workers aboard Chasm Forge. (TV: Oxygen)

Skills and abilities
Nardole was capable of flying the TARDIS, a talent displayed by very few of the Doctor's previous travelling companions. (TV: The Return of Doctor Mysterio)

Following his reassembly at least, Nardole had a longer-than-human lifespan. He remained at St Luke's University for a number of decades during which he didn't appear to age.

He was also adept at using a sonic screwdriver. When they travelled to a Dalek-Movellan War, he used it short-circuit several panels and successfully quarantined all the Daleks in the vicinity to prevent them from attacking the Twelfth Doctor and Bill. (TV: The Pilot)

Appearance
Nardole resembled a short, hairless, humanoid male with a rotund build. He claimed that he hadn't seen his true face in years and that he swapped it out for his current face when on the run. (TV: Oxygen)

On Mendorax Dellora, Nardole wore a crimson duffle coat matched with a multi-coloured beanie to cope with the snowy climate. (TV: The Husbands of River Song) He continued to wear the same coat when travelling with the Doctor (TV: The Return of Doctor Mysterio, The Pilot, Oxygen), but also wore a plaid red vest with square glasses when assisting him at St Luke's University, (TV: The Pilot, Smile, Thin Ice, Oxygen) or a red lumberjack coat. (TV: Knock Knock)

After Nardole ruled 12th century Constantinople, he was seen wearing imperial garb appropriate for the period. (TV: The Return of Doctor Mysterio)

Behind the scenes

 * Several characters in Doctor Who Legacy are based on Nardole, including Hydroflax with Nardole's head, which is used as both an ally and an enemy.
 * Nardole is mentioned by the Twelfth Doctor in Doctor Who and the micro:bit.