Master Xehanort is a main character in the Kingdom Hearts series. In a way, he appeared in every game so far, but only in a variable form. He is the main antagonist of the Dark Seeker Saga.
He is old and has a very short white hair and some beard. He is not tall, but he decided to give into the darkness, so he could try to fulfill his ambitions; he wanted to see what would happen if another Keyblade War would be created.
 Before Kingdom Hearts Birth by Sleep
Xehanort was born at Destiny Islands. When he was a teenager, he was greeted by his Heartless from the future to grab twelve vessels through time. When he returned to the present, he lost memory of the event. He went to the Land of Departure to become a Keyblade Master and he abandoned his Keyblade Armor and made an alternative. Through the years, he found the story of the Keyblade War and the X-blade, and slowly became obsessed with it. He resented his classmate Eraqus and gave him two scars from his keyblade. He recruited Ventus in an attempt to create the X-blade. This however, failed, and Xehanort extracted his negativity into a man. Xehanort let Ventus to rest at Destiny Islands. Ventus was woken up by a new born's heart, and Xehanort let Ventus get trained by Eraqus like Aqua and Terra. Unfortunately, Ventus lost his memory.
 Kingdom Hearts Birth by Sleep
Xehanort watched Aqua and Terra take the Mark of Mastery exam, however, Xehanort had added darkness inside Terra. Aqua had won the Mark of Mastery because of this, and Xehanort told Terra to never fear the darkness. At the Keyblade Graveyard, Xehanort told Terra about Vanitas and Radiant Garden. Xehanort told Maleficent, at the Enchanted Dominion, about the Princesses of Heart. Xehanort was acting that he got kidnapped by Braig at Radiant Garden., and after Braig battled with Terra, he became Xehanort's ally, and Xehanort gave some of his heart to Braig. Xehanort met Ventus again at the Graveyard and told him about the X-blade. Terra and Aqua met Xehanort soon after, and Xehanort led Aqua and Terra to the Graveyard. When the X-blade was forged by Ventus and Vanitas, Xehanort decided to stab himself with his Keyblade to let his heart rest inside Terra.
 Between Kingdom Hearts Birth by Sleep and Kingdom Hearts II
Xehanort's fusion, Terra-Xehanort, worked with Ansem the Wise, Braig, and the other apprentices. Xehanort worked on experiments until Ansem forbid them. After Xehanort sent Ansem to the Realm of Darkness, they worked on experiments with each other. Xehanort stabbed Braig with his keyblade causing Braig's heart to be removed and Xehanort taking Ansem's name. Xehanort was then split into a Heartless and a Nobody, which had part of Xehanort's plans. Ansem was intended to help Maleficent snag the Princesses of Heart and expand the normal Kingdom Hearts by ravaging worlds with darkness. Xemnas intended to divide Xehanort's heart into thirteen, but the goal was impossible and Xemnas' group changed the intention to make a Kingdom Hearts out of the hearts of men. Both were defeated by Sora ,but this caused Xehanort to be revived.
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