Heidi (The Girl Who Died)

An unnamed Viking, nicknamed "Heidi" by the Twelfth Doctor, was a Viking who was trained to be a warrior to fight the Mire. He fainted at the sight of blood.

Biography
After the Mire abducted the village's warriors and Ashildr had declared war on them, "Heidi" was one of the ones who was chosen to train with the Doctor to become a true Viking warrior. He was surprised when the Doctor gave him the name 'Heidi' instead of his real name. He revealed to the Doctor that he fainted at the sight of blood. He later fainted during training and quickly recovered, but when Clara mentioned blood he fainted again.

Later, when Odin arrived with the Mire in tow, "Heidi" was partying with the others when (according to the Doctor's plan) 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. (TV: The Girl Who Died)