Einarr

Einarr, nicknamed "Chuckles" by the Twelfth Doctor, was a Viking farmer who was trained to be a warrior to fight the Mire.

Einarr's village was visited by the Doctor and Clara Oswald during the 9th century, coinciding with an attempted incursion by the Mire. The Mire transported the warriors of the village, along with Clara and Einarr's daughter Ashildr, to their ship; Einarr was devastated at the thought of losing her, and relieved when she returned unharmed.

Though initially angered by the Doctor's insistence that the Vikings abandon their village rather than fight back against the Mire, Einarr agreed to train with a sword under the Doctor's tutelage. He came to perform a key role in the Doctor's plan to defeat the Mire, using electric eels to generate electricity which was in turn used to play havoc with the Mire's equipment.

Einarr was heartbroken when Ashildr died defending the village; it was, in part, his grief which inspired the Doctor to save her life using a modified Mire medical kit (with the unintended side-effect of making her immortal). Shortly after Ashildr was resurrected, the Doctor gave Einarr a second kit, for Ashildr to use on whoever she wished to share her newfound immortality. (TV: The Girl Who Died)

Centuries later, in 1651, when the Doctor mentioned Einarr to Ashildr, she could not remember him. (TV: The Woman Who Lived)