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)

Following twenty-four years on Darillium, River went to her death in the Library and the Doctor reassembled Nardole out of fear of being alone. (TV: The Return of Doctor Mysterio)

Working for River Song
Nardole first encountered the Doctor when 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)

Travels with the Doctor
After spending twenty-four years with River Song, the Twelfth Doctor feared being alone, and so returned to Nardole and reassembled him, taking his head from Hydroflax and "gluing" it back on. Nardole became his companion and saw it as his duty to make sure that the Doctor was alright following her death.

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 spent a number of decades at St Luke's University in Bristol, where they watched over a secret vault hidden beneath the campus. While stuck on Earth, the Doctor took a job as a lecturer and Nardole acted as his valet. (TV: The Pilot)

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)

He was timid and could show great fear when faced with danger. He was particularly scared of heights. (TV: The Husbands of River Song)

Although he could sometimes appear incompetent, (TV: The Husbands of River Song) he was knowledgeable and, despite not originating from Earth, knew that New York wasn't a capital city, unlike Washington DC.

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. (TV: The Pilot)

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.