Einarr

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

When his daughter Ashildr was kidnapped by the Mire after they locked onto the sonic sunglasses, Einarr was devastated and angered by the Doctor's persistant rudeness. Later, when Ashildr returned, she confessed that she had spoken for the village and declared war on Odin and the Mire.

He was trained by the Doctor to fight with a sword, but he eventually came to terms with the Doctor's plan of using electric eels to generate electricity to defeat the Mire.

Later, when Odin arrived with the Mire in tow, Einarr activated the electric eels and Ashildr, through a Mire helmet, projected the image of a dragon attacking the Mire. This scared them off, and the village celebrated over their retreat. However, Einarr became instantly devastated again since the electric charge through the helmet had killed Ashildr. However, the Doctor used Mire technology to repair her so that she was resurrected. He gave Einarr another chip for Ashildr to use to keep someone with her and share her immortality. (TV: The Girl Who Died)

Centuries later, in 1651, the Doctor mentioned Einarr to Ashildr, for whom he had become a distant memory. (TV: The Woman Who Lived)