The Asgardian known as Mimir was the child of Buri and brother of Bor, father of Odin and the Asgardians. Mimir was a long time enemy of his nephew Odin who transformed him into a fiery being at one point. Thereafter, Mimir existed mostly as a disembodied head that dwelled in the Well of Wisdom in Asgard.
He was given by Odin to the Vanir as a peace hostage during the Aesir-Vanir War. For unknown reasons, the Vanir decapitated him and send him living, talking head back to Odin.
Odin sacrificed his right eye to Mimir for the wisdom to prevent Ragnarök. Mimir was a virtually omniscient being with precognitive abilities. Thor often traveled to Hildstalf, to seek out the wisdom from the Well of Mimir. Thor sacrificed both his eyes for insight on how to permanently stop Ragnarok. Mimir was shown to have been killed in the destruction of Asgard by the Thunder God himself.