Yu Yevon

Yu Yevon is the true antagonist of Final Fantasy X. He is the creator of Sin and the so-named founder of Yevon, the cult controlling Spira (In truth, the cult is only named after him- what it was in fact created after the misguided destruction of his body) which teaches people to go on a pilgrimage to destroy Sin while sacrificing themselves, only for Sin to return later on. Every time this happens, the person used to create the "Final Aeon" that defeats Sin becomes the host for the new incarnation of Sin, while Yu Yevon remains the true controlling force inside Sin. He was also the father of Lady Yunalesca. Yu Yevon was the leader of Zanarkand 1000 years ago but during a battle with Zanarkand and Bevelle, Yu Yevon knew Zanarkand would fall so he turned the citizens of Zanarkand into Fayth and created a Dream Zanarkand under the sea. Yu Yevon then created a powerful armor out of many pyreflies and thus created Sin which destroyed both Bevelle and Zanarkand and had plagued Spira in a spiral of death ever since.

Yu Yevon's existence is not revealed until late in the story, and he doesn't appear until the very end after the heroes defeat Jecht, the current host of Sin in the form of Lord Braska's Final Aeon. Yu Yevon then reveals himself as a dark glowing creature with the symbol of the Yevon cult on it, and with all his defenses in the form of Sin bypassed, he is very weak. His only attack is the Gravity spell, which automatically takes away half of the target's remaining HP and thus can't actually kill the characters, and he is killed by Tidus, Yuna and the others, destroying Sin once and for all. With Yu Yevon gone, Dream Zanarkand fades away as does Tidus. All Fayth are then able to find eternal rest, until Vegnagun attacks the farplane in the second game.

Trivia

 * The Yu Pagodas combine together around Yu Yevon, and then they all dissolve in the same way as Chaos after his defeat. This is the first time this style of death is used since Safer∙Sephiroth in Final Fantasy VII.
 * Every time Yu Yevon possessed one of Yuna's Aeons during the end of the game, they took on the appearance of the Dark Aeons. This also suggests that Braska's Final Aeon looked different in terms of coloration before becoming Sin's core.
 * The original plot draft for Final Fantasy X indicated that portraits of Yu Yevon were displayed in public places, but this was scrapped in the final version.
 * It is possible to defeat Yu Yevon in two turns in the International Versions by first using Zombie Attack, then using Life.
 * Lady Yunalesca is his daughter.
 * The Fayth in Bevelle has stated that Yu Yevon is neither good or evil.