Do you think I'm going to beg for forgiveness? Ridiculous! There is no Goddess. So I will continue to pursue my ideals. If there is no place where I can live, and if I've been denied my Age of Lifeless Beings... then the only thing left for me is to build a new world on Derris-Kharlan. A world just for my sister and me!
~ Mithos before the final battle

Mithos Yggdrasill is the main antagonist in Tales of Symphonia. His "Mithos" voice was done by Minami Takayama in Japanese and English dubbed by Brainee Siddall; his "Yggdrasill" voice was done by Hideyuki Tanaka in Japanese and English dubbed by James Arnold Taylor. He is the leader of the Cruxis and Desians that wanted to ressurrect Martel and rid the world of discrimination against half-elves.

In Tales of Symphonia

After Remiel died and that Kratos first betrayed Lloyd and the others, Yggdrasill appeared before them and demonstrated his own power against them. Later, he changed his form into a kid and simply nicknamed himself "Mithos". He acted as Genis's "best friend" and even proved that by using his flute that was a momento of his sister "Martel". However, he eventually revealed his true nature by attacking Yuan and two renagades.

At the Tower of Salvation, he snatched away Colette in order to bring back Martel. Realizing that Martel told him to stop what he was doing, Mithos Yggdrasill didn't listen to her words. After coming back to life after his first defeat, it was because that his life force kept coming from his Cruxis Crystal. After Lloyd's friends fell into a trap, Mithos used illusions to frustrate them. However, Lloyd and Colette were able to bail them out. At Mitho's Castle, Mithos engaged in the final battle with everyone. Later in the battle, he went back to his adult form and had a large entity around himself. In the end, while Mithos faded away, Lloyd destroyed the Cruxis Crystal, killing him for good.