Mithos Yggdrasill acts as both the main villain of the game and a primary hero in its story. He is first mentioned as Mithos the Hero, a legendary man who lived four thousand years before the beginning of the game. He does not physically appear in the story until the player is about a third of the way into the game. Even then, it is not revealed that he is actually Mithos the Hero himself; this is left unknown until the last third of the game. Mithos is best known for his belief that he is a hero and is saving the world, his childish appearance, and even his successful infiltration of the player's party.

As is made clear in the game, Mithos had strong ideals about peace and unity, but his sister's death deepened his hatred for humanity and made him lose track of whether the world, or her life, was more important to him. He took her final words, a wish for a world free of all discrimination, to heart, misinterpreting it as a wish to end the persecution of only those who were half human and half elf. Mithos' ideals then became twisted: he became obsessed with resurrecting his sister and with ushering in a new "Age of Lifeless Beings" by using Exspheres to make everyone the same.

Despite the fact that he truly means well and believes that he is saving the world, Mithos creates enemies everywhere: even his former companions, Kratos and Yuan, turn against him.

(Source: Wikipedia)