Hel (Old Norse Hel, “Hidden”) is a giant and goddess in Norse mythology who rules over Helheim, the underworld where the dead dwell. According to the thirteenth-century Icelandic scholar Snorri Sturluson, she’s the daughter of Loki and the giant Angrboða (“Anguish-boding”), and therefore the sister of the wolf Fenrir and the world serpent, Jormungand.
Fenrir and Tyr, Norwegian folklore. Okay, not medieval per se, but dark ages! I'm partial to this one: a warrior god who seeks to bind the monstrous wolf, Fenrir, trying to keep the world from the trouble the beast was foretold to cause, and who gets bit on the hand.