Mimir was originally the son of Buri and brother of Bor and Njord, during the Aesir-Vanir war, Mimir was given as a peace hostage to the Vanir, and then they decapitated him for motives who are unknown, he later would become enemy with his nephew Odin after he was transformed into a fiery being by Odin, living as a disembodied head that lives within the Well of Wisdom at the base of Yggdrasill in Asgard.
Mimir often appears when someone needs information or advice. The fountain of knowledge has incredible post and precognitive abilities, as "there is naught that may be kept secret" from him. Both Odin and Thor have greatly sacrificed to Mimir for knowledge. Odin gave his eye for the wisdom needed to stop Ragnarok and Thor, needing to better his father for the same knowledge, gave both his eyes.