Mithos is introduced under his surname as an adult, but is able to interchange into his child body since his appearance and growth is controlled by his Exsphere. He was originally a kind hearted person despite the discrimination he faced for being a half elf. Four thousand years prior to the game, he and his companions, Martel, Kratos Aurion, and Yuan, ended the war between the countries Sylvarant and Tethe'alla by using the Eternal Sword and splitting the two into different worlds. His plan was to reunite the worlds when the comet known as Derris Kharlan approaches Earth so he can use its mana to germinate the Great Seed into a Giant Kharlan Tree. The humans attempted to monopolize the Great Seed and killed Martel in the process. Daunted by her death, Mithos delays the germination and instead places Martel's soul inside the Great Seed until he can find a new body for her. Her death caused him to gain a strong hatred towards the world and becomes cruel and merciless. Years later, Mithos forms the organization known as Cruxis in order to find a compatible body for Martel and to fulfill his Age of Lifeless Beings plan, a plan where everyone's consciousness will live inside their Exspheres free from their body; the latter being a twisted interpretation of Martel's last wish. Mithos' ideas resulted in the deaths and suffering of many people and the party is forced to kill him.
In Super Smash Flash 2
Mithos Yggdrassil is the hazard in the stageTower of Salvation in Super Smash Flash 2. He hovers around the top of the stage and attacks the players with special powers according to the Tales series, including Volt Arrow or Death Kneel. Players are also able to attack him, and when attacked, he counters with a punch forward that deals about 8% damage.
Lloyd attacks Mithos with Tempest; while Mithos use counter-attack Lloyd, but missed.