Mímir is the keeper of the well Mímisbrunnr which is located deep beneath one of the roots of Yggdrasil. The name Mímir comes from the ancient Norse meaning "the rememberer" or "the wise one". He is theorised to be the unnamed brother of Bestla from whom Odin gains wisdom. This would make his father Bölþorn.
Mímir discovered the well and used the mighty horn Gjallarhorn to take a drink from it gaining the wisdom of ages.
Odin visited Mímir seeking to take a drink from the well to gain its wisdom and was required to pluck out and leave one of his eyes in order to earn the drink.