Yu Yevon

Yu Yevon is the true villain of Final Fantasy X. He is the creator of Sin and the founder of the cult controlling Spira 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'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 his 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.