Mimer “The wise one or The rememberer” is known for his knowledge and wisdom in all the nine worlds. During the war between the Asir and Vanir, Mimir was sent to the Vanir along with the God Honir as part of a peaceful hostage exchange.
But because the Vanir was fearing trickery from the Aesir, the Vanir beheaded Mímir and sent his head back to the Gods and Goddesses in Asgard. In order to keep his wisdom, Odin preserved his head with magic so it could continue to provide Odin knowledge and counsel as his advisor.
He has his own well called Mimir’s well “Mímisbrunnr”, those who drink from the well will obtain more wisdom. Odin once drank from the well, and paid for it, by sacrificing an eye to Mimir. This well of wisdom is located by the second root of the tree Yggdrasil.