Mithos Yggdrasill serves as both the central antagonist and a key heroic figure in the game's narrative. Initially, he is referred to as Mithos the Hero, a legendary figure who lived four thousand years before the events of the game unfold. He does not make a physical appearance until the player is about one-third through the game. Even then, it is not revealed that this character is indeed Mithos the Hero; this revelation is withheld until the final third of the game. Mithos is renowned for his belief that he is a hero and savior of the world, his youthful appearance, and his successful infiltration of the player's party.
Mithos originally held strong ideals about peace and unity. However, his sister's death intensified his hatred towards humanity and caused him to lose sight of what was more important to him: the world or her life. He took her last wish—a desire for a world free from all discrimination—deeply to heart, but misinterpreted it as a wish to end the persecution of only those who were half-human and half-elf. This led to a twisted transformation of Mithos' ideals. He became obsessed with resurrecting his sister and initiating a new "Age of Lifeless Beings" by using Exspheres to make everyone identical.
Despite his genuine intentions and belief that he is saving the world, Mithos ends up creating enemies everywhere, including his former companions, Kratos and Yuan, who turn against him.
