Mimir is well-mannered, kind and possesses a cheery if not sarcastic and witty sense of humor. Even in his beheaded state, Mimir tries to make the most of the situation, finding it better than imprisonment and as such he tries to be helpful in any way he can. Mimir often has a wealth of information regarding the deities, monsters, civilizations, the Nine Realms and famous figures of the Norse mythology. Mimir also uses to end most of his phrases with the word "brother", possibly as a form of camaraderie to Kratos and his son Atreus, whom he sometimes calls "little brother" or "lad".
Hailing to from a faraway land, Mimir was a faerie king's errand boy and unofficial jester. Frequently, Mimir and other faeries, nicknamed "Goodfellows", would walk amongst mortals to sow mischief and as long as his lord was kept amused, they were spared the consequences. Eventually, Mimir's king grew weary of his antics, forcing him to leave his homeland.
After many years traveling north, Mimir eventually ended up in the Nine Realms and learned of Odin, king of the Aesir. Determined to prove his worth, Mimir approached Odin and provided him with a supposed mystical well of knowledge, which was actually a well of water laced with enough mystic mushrooms powerful enough to make a god see visions. Despite Odin tearing out one of his eyes after imbibing of the hallucinogen, Mimir claimed the Allfather was bestowed with greater sight and was hired as Odin's primary adviser and ambassador to all the other realms. Unlike the Aesir, Mimir sought to establish peace between the realms, as he believed it was the best plan to avert Ragnarok. However, Odin rarely listened to such advice and often led many wars, particularly with the Vanir and the Giants. When the war with the Vanir caused unprecedented devastation and left both sides weary, Mimir suggested a truce: Odin would marry his deadliest enemy, the leader of the Vanir, the Goddess Freya. After much convincing, Odin and Freya agreed to the terms, thus ending the war.
Overtime, Mimir grew to befriend many Giants from Jotunheim to the point where he was bestowed with Bifrost crystals in his eyes. Because Mimir spent so much time traveling through realms, they believe it would be more convenient than a crystal he could lose. To numb the pain of the procedure, Mimir drank sixteen cups of billow maidens' ale. So inebriated, Mimir almost convinced the Giants to put the crystals in his nipples instead, jokingly calling himself, "Mimir of the Bifrost Teats".
However, Mimir's efforts to preserve peace would be stymied by Odin's paranoia, his obsessions with prophecies and allowing his son, Thor, to go on a killing spree on the Giants. Thus, much like with Tyr, Odin confronted Mimir, suspecting that he aided the Giants. As punishment, Odin imprisoned him on Midgard's highest peak, bound to a tree made indestructible even to Thor's hammer. As additional punishment, Odin removed the horned god's bejeweled left eye and hid it to keep Mimir from traveling to other realms; Mimir then realised that Odin had never been fooled from the drugged well incident since they met. Until he met Kratos, Mimir was tortured by Odin himself on a daily basis. This eventually made Mimir yearn for death.