N is for... Nardole

The Doctor first met Nardole on the Earth colony world of Mendorax Mellora, at Christmas 5343. He had been sent by his wife to fetch a medic whom she had arranged to meet here. Due to a mix-up, the Doctor thought that it was he whom Nardole sought, whilst he believed the Doctor to be the medic.

Taken to a crashed spaceship on the edge of the village where the TARDIS had materialised, the Doctor was shocked to discover that Nardole's spouse was River Song. Not only that, but he was supposed to help treat another husband - the brutal King Hydroflax. Only his head remained of him, attached to a robot body. An explosion had resulted in a precious gem being embedded in his skull, which River wished removed. She did not recognise the Doctor in his twelfth incarnation.

River actually only wanted to steal the gem, and she and the Doctor ended up taking the King's head. The semi-autonomous robot body decapitated Nardole to use as its new head. The same fate befell a third husband of River - Ramon. The robot would swap heads over time, with the unused one continuing to function within its torso. It would later come to be employed as a waiter in a restaurant beside the famous Singing Towers of Darillium.

The Doctor later saved Nardole from this fate, building a makeshift artificial body for him. 

The pair travelled together for a time, and at one point Nardole actually ruled the early Byzantine Empire.

However, River had given Nardole a role to perform before she and the Doctor parted for the final time.

He was tasked with guarding a vault in which Missy was to be incarcerated for a thousand years, after preventing her execution. The Doctor hid this vault in the basement of St Luke's University in Bristol where he took on a teaching role. Nardole joined him there, acting as his valet but also to ensure that he adhered to his vow to oversee the vault.

It was in his role as valet that he first met canteen assistant Bill Potts, whom the Doctor agreed to tutor.

Frustrated with being tied to one place and time for so long, the Doctor wanted Bill to travel with him and see the stars. Nardole disliked her for making the Doctor want to go against his vow, and attempted to stop their travels - but to no avail. He even tried to sabotage the TARDIS by removing a fluid link, but the Doctor predicted his actions.

Instead, he decided to go with them so that he could ensure that the Doctor remained safe and able to go back to his responsibilities at the university.

Their first journey was forced on them, as Bill was stalked by a sentient water-borne AI which resembled her potential girlfriend, and now wanted to travel through space with her as her co-pilot.

Over time, Bill came to learn that Nardole had led an extremely interesting life himself - often engaging in illegal activities such as smuggling and black-marketeering. It was his efforts to escape a life of crime which had led him to being employed by River Song, whom he subsequently married.

When the Doctor became blind after being exposed to the vacuum of space, outside the Chasm Forge asteroid mining station, he kept this secret from Bill but told Nardole, who naturally challenged him about the dangers of abandoning their mission to guard the vault.

However, when the Doctor and Bill became stranded on Mars in Victorian times, Nardole actually opened the vault to allow Missy to pilot the TARDIS back to the Red Planet to rescue them.

The Doctor attempted to rehabilitate his old enemy, allowing her to accompany them on their travels - much to Nardole's concern. On encountering a tribe of Picts in 2nd Century Scotland, Nardole managed to be accepted into the community as a story-teller.

Nardole's travels with the Doctor came to an end when they visited a vast colony ship trapped on the edge of a Black Hole. This originated on Mondas, and they witnessed the evolution of a new race of Cybermen. Missy also encountered a previous incarnation of herself. Nardole befriended a woman named Hazran who looked after a group of orphaned children, protecting them from encroachment by the Cybermen.

With the vault no longer needing guarding, the Doctor gave Nardole a new role - to continue to protect Hazran and the children as they moved to a new home within the vessel.

Just before his next regeneration, the artificial intelligence Testimony allowed the Doctor to see a glass avatar of Nardole, holding all his memories, for a final time, along with similar duplicates of Bill and Clara Oswald.

Played by: Matt Lucas. Appearances: The Wedding of River Song (2015) to Twice Upon A Time (2017).

• Lucas first came to public attention when he featured in several series with surreal comics Vic Reeves & Bob Mortimer, before finding fame with David Walliams in Little Britain. 
• He and Walliams were huge Doctor Who fans, leading Little Britain narrator Tom Baker to declare that he was now being employed by the children who had grown up with his Doctor.
• The actor was originally intended as guest artist on the 2015 Christmas Special only, but everyone enjoyed his presence so much that Steven Moffat brought him back the following year in The Return of Dr Mysterio. Similarly, a brief appearance in the 10th series was expanded so that he featured throughout. Lucas was living abroad and working on his autobiography at the time.
• The Target novelisation of Twice Upon A Time claims that Nardole spent the rest of his life on the colony ship, defending everyone from annual attacks from the Cybermen. He died aged 728, after marrying six times.