"Heidi" was the nickname given by the Twelfth Doctor to a Viking who was trained to be a warrior in preparation for a battle with the Mire. He suffered from haemophobia, fainting at the sight of blood.
After the Mire, whose leader was posing as Odin, abducted the village's warriors and threatened to return to finish off the rest of its inhabitants, "Heidi" was one of the villagers chosen to train with the Doctor to become a true Viking warrior. He revealed to the Doctor that he fainted at the sight of blood. He later fainted during training and quickly recovered, but when Clara Oswald mentioned blood he fainted again.
The Mire returned the following day, but were defeated by the villagers thanks to a plan constructed by the Doctor which involved using electric eels to generate electricity, which in turn would be used to play havoc with the Mire's equipment. "Heidi" celebrated with the rest of his village after the Mire retreated. (TV: The Girl Who Died)