|“||Some people value their privacy. Best not to judge, brother.||”|
–Mimir to Kratos
Mimir was imprisoned by Odin before the events of the game. Up until he met Kratos, he was tortured by Odin himself on a near-daily basis. Baldur confronts him and attempts to bribe him with freedom in exchange for revealing Kratos and the boy's whereabouts, but Mimir was unable to locate them due to a concealing rune placed on the pair by a witch. Even if he could, Mimir knew that Odin would never release him, no matter what Baldur would say to his father. As such, Mimir demanded the trio to leave.
Almost immediately after Baldur leaves, Kratos and Atreus reveal themselves to Mimir. Mimir introduces himself to the two. When Atreus describes their journey to the tallest peak in all the Nine Realms, Mimir states that Atreus is mistaken about choosing Midgard's tallest peak, as the tallest peak in all of the realms is in Jötunheim. He also reveals that he can open the last known gate to the realm.
In return, Mimir asks Kratos to cut off his head, explaining that after over 100 years of torture, his current state is not living at all. Kratos agrees to do so. Atreus cannot bear to watch and leaves, and when he does so, Mimir tells Kratos he must reveal his past to Atreus and the longer he waits, the more likely Atreus will grow resentful of him. Kratos then cuts off his head, temporarily killing him.
His lifeless head is brought over to Freya's home, where she revives it. It is quickly revealed that the two have a bad relationship, evidently after Freya spat in his face. Mimir also reveals Freya's identity, and in return learns that she never revealed it to Atreus and Kratos. Freya warns Kratos that Odin's wrath will be coming quickly now that Kratos has Mimir, and Kratos storms out of Freya's home.
Mimir then becomes part of the traveling company of the Spartan and his son, lending his considerable knowledge of the realms, insight on the depraved, violent nature of the Aesir gods and of the tragedies caused by wars throughout the ages. He also tries to mend the strained relationship between the father and the son.
When he is first found, Mimir is shown to be in a tree. He has two small horns on his head and a growing, grey beard. He is missing one of his glowing, golden eyes. He eventually is decapitated and brought back to life, though he still looks the same. Later, Kratos and Atreus retrieve Mimir's other eye, and put it back in his head, and find that it still works.
Mimir is well-mannered, kind and possessed 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. 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. Being a part of the traveling company of Kratos and his son, Mimir becomes something of a balancing force between them, teaching Atreus to use his abilities for good and often advising Kratos to be more open about the truth of his past. To that end, Mimir demonstrates that he is emotionally sensitive and insightful to the potential impact that Kratos's distant attitude towards Atreus could have.
Quickly, Mimir proved to be a loyal ally and friend to Kratos and his son, helping them find the realm of the giants all the way. He even begged Baldur to stop attacking Kratos and Atreus, offering to do whatever he wanted but of course, his pleas fell on deaf ears.
Despite his bright personality, Mimir possesses a great deal of anger and resentment towards the Aesir gods, Odin and Thor in particular. He finds a great majority of them to be controlling, paranoid, arrogant and the ones to blame for the tragedies the Nine Realms have endured. Before his imprisonment, Mimir did his best to try to bring and make peace between the realms wherever he could, such as advising Odin to marry Freya to end the conflict between the Aesir and Vanir gods.
In contrast to the Aesir gods, Mimir held Tyr in the highest esteem, believing him to be a great leader who fought for peace, knowledge and understanding, not for power and control.
Even though he is part of the Norse mythology, Mimir is aware of other mythologies and their pantheons, mentioning in Tyr's vault about the Egyptian gods and of the Greek pantheon in Helheim. With the Greek gods, he also knows of their demise, which he claimed was a long time coming, however, he never knew who did it until Kratos took him along.
- Mimir and Helios were the only gods that Kratos decapitated. Also, both of them had a use for him.
- Mimir's imprisonment and initial death are similar to Prometheus's situation, as both were imprisoned by the ruling god of their land for a supposed crime, and both were mercy killed.
- Mimir was stated to be a very close friend if not a lover to the Valkyrie Queen Sigrun.
- Mimir is one of the few individuals who know and can speak the language of the giants. Thus, he is one of the few who can speak with the World Serpent.
- Mimir figured out Kratos was Greek due to him saying Athena's name. Later on, Mimir finds out that Kratos was the son of Zeus which explained a lot to him and soon causing him to exclaim that Kratos is the Ghost of Sparta.
- Mimir is the only god who genuinely helps Kratos in his journeys without even trying to use him for some selfish purpose, which would eventually lead to some kind of betrayal, and even going as far as to call him "brother". Ares, Athena, and the Olympians, on the other hand, merely used him as a killing vessel for their own personal gain while Hephaestus is initially helpful until he turns against Kratos to protect his daughter.
- His Greek equivalent is Athena and Mnemosyne. He replaced Athena as Kratos' personal ally in wisdom and knowledge from this point onward.
- While in Tyr's secret room, Mimir stated that the Sgian-dubh is from his homeland which would mean he is Celtic rather than Norse.
- Mimir greatly resembles , a deity in Chinese religion, a mythical sage ruler of prehistoric China.