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Contain big spoilers, took from wikipedia.
Kratos Aurion "Kuratosu Aurion"
Japanese voice actor: Fumihiko Tachiki
English voice actor: Cam Clarke
Age: 28 (real age: over 4000)
Height: 186 cm
Weight: 78 kg
Kratos is a mysterious mercenary who is hired to accompany Colette on her journey. Though he accompanies the party for quite some time, it is later revealed that Kratos is a high-ranking angel, one of Mithos Yggdrasill's from the Ancient Kharlan War, and a spy for Cruxis who was sent to ensure that Colette completed her task. He betrays the party in the Tower of Salvation, but his true motives remain unclear until much later, when it is revealed that he is actually Lloyd's father and has been searching for a way for a human to wield the Eternal Sword and fix what Mithos had done to the world. He was forced to kill Anna, his lover and Lloyd's mother, when she was turned into a monster by an uncontrolled Exphere.
Despite his long life and many years of experience, Kratos is, up until the end of the game, a fatalist, taking a long time to come to the realization that he does not have to cling to Mithos' ideals. Depending upon choices the player makes, he may or may not rejoin the party at an extremely late point in the game. He hates himself to a good extent, and constantly tells Lloyd not to die, probably because he never truly got to be a father to Lloyd. Kratos is a powerful melee fighter and an excellent spell caster. He wields a double-edged blade and shield, and is proficient with elemental, curative, and angelic magic.
Mithos Yggdrasill "Mitosu Yugudorashiru"
Japanese voice actors: Minami Takayama (Mithos form) /Hideyuki Tanaka (Yggdrasill form)
English voice actors: Brianne Siddall (Mithos form) / James Arnold Taylor (Yggdrasill form)
Age: 14 (real age: over 4000)
Height: 153 cm
Weight: 41 kg
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. According to the legend, Mithos and his three companions stopped the Ancient Kharlan War and restored peace to the world. Mithos's full name is not revealed until extremely late in the game, along with the identities of his companions: Kratos Aurion, Mithos's sister Martel, and Yuan. These companions would later be referred to as the Four Seraphim, the leaders of Cruxis; the three surviving members are over 4,000 years old. It is later revealed that, during the War, the companions went through many great hardships due to rampant discrimination against half-elves, fueled by both humans and elves. Because of this discrimination, Mithos and Martel were shunned from their home of Heimdall and were forced to live on their own. During this time, Mithos was trained as a swordsman and a mage by Kratos. He would eventually go on to become the great hero of legend by single-handedly making pacts with each of the Summon Spirits. Mithos then had Origin create the near-omnipotent Eternal Sword, which he used to split the world in two (thereby creating the parallel worlds of Tethe'alla and Sylvarant) in order to prevent more magitechnology from being created. The Ancient Kharlan War had been caused by magitechnology, which consumes vast amounts of mana; by splitting the world in two and controlling each half's access to mana, Mithos effectively stopped the creation of magitechnology and a second cataclysmic global war. Unfortunately, his beloved sister was slain during the conflict.
Mithos had strong ideals about peace and unity, but his sister's death deepened his hatred for humanity, and he began to obsess over her loss. He took her final words, a wish for a world free of all discrimination, to heart, but misinterpreted them as a wish to end the persecution of only half-elves. 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. Part of the pact with Origin was a promise that he would restore the Great Seed to the world so that it could grow again; this would restore mana to the land. Instead, Mithos took it to Derris-Kharlan in order to revive and protect his dead sister, thereby breaking his pact with the Summon Spirits.
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. At a very late point in the game, in the Hall of the Great Seed, Mithos finally manages to revive Martel by channeling her consciousness through Colette. Contrary to Mithos' expectations, the awakened Martel expresses deep regret and disappointment at what her brother has done. She begs him to stop creating his Age of Lifeless Beings, saying that what he has been doing "is wrong." Mithos sees this as the ultimate betrayal; it drives him to a state of total denial. Outraged, he attacks the party and loses, his body vanishing into his Cruxis Crystal. His spirit later escapes the Crystal and flees to Derris-Kharlan, which he tries to separate from the worlds; he is pursued by the party. Mithos' spirit (within the Crystal) reaches out to each of the party members in a desperate attempt to get them to join him. One by one, the party rejects him, pledging to risk everything in order to stop him. When Mithos is finally defeated and killed in the throne room of his fortress of Vinheim Castle, he tells the party that, despite all, he feels no regret for the choices he made, and, if given the chance, he would do everything all over again in order to save his sister. He then orders Lloyd to destroy his crystal before he has that chance. Lloyd does.
