Nardole

Nardole was a member of the human colony of Mendorax Dellora, and an employee of River Song.

He was told to find the surgeon Professor Song needed to extract the Halassi Androvar from the head of King Hydroflax. Unfortunately, Nardole found the Twelfth Doctor and brought him to Professor Song instead.

When River and the Doctor escaped Hydroflax's ship with his head in a bag, Hydroflax's mechanical body turned to Nardole for strategic information. Although Nardole claimed to know 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. Holding a gun to Nardole's head, the 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)

After 24 years with River Song, the Twelfth Doctor later repaired Nardole, taking his head from Hydroflax and "sewing" it back on.

From that point he became a companion of the Twelfth Doctor. They together investigated Harmony Shoal with the help of the Ghost, Grant Gordon. (TV: The Return of Doctor Mysterio)

Nardole