He who crafts the souls of the dead into unholy armor. An armor called Sin. Clad in it, Yu Yevon is invincible.Grand Maester Mika
A mysterious entity, the only information given about Yu Yevon comes from Bahamut's fayth, Yunalesca, Maechen, and Grand Maester Mika. Yu Yevon's role is that of an impersonal antagonist; while Jecht and Seymour fill the roles of personal antagonists for Tidus and Yuna, Yu Yevon has no personal connection with any of the main cast.
Yu Yevon has no dialogue in his single appearance, so his personality is unknown. Accepting the doom of the original Zanarkand, he set forth in creating and maintaining its memory through the surviving citizens. In some respects, he can be considered a tragic character as he is incapable of stopping what he is doing. Ultimately, Yu Yevon has become indistinguishable from Sin itself.
His appearance is a mystery, as he is never shown in human form. Yu Yevon appears as a bizarre, floating creature with hooks and a glowing emblem of Yevon for a face. The nature of this form is unexplained. It may be that transplanting his soul from each Sin to its successor has diminished or destroyed his true physical form, leaving him in a parasitic, bug-like state. Yu Yevon also may have taken on this form intentionally, perhaps to facilitate his ability to possess aeons. Another possibility is this form is his appearance as a fiend; this is unlikely though, as he retains the ability to summon Dream Zanarkand. He also briefly appears as a floating ball of light similar to a pyrefly to signify his ethereal form.
Yu Yevon was once a summoner, long ago. He was peerless. Yet now he lives for one purpose: only to summon. He is neither good, nor evil. He is awake, yet he dreams. But... maybe not forever.Fayth
One thousand years ago Yu Yevon was the leader of Zanarkand and a powerful summoner. Zanarkand was at war with Bevelle, a strong military power that used mechanical technology to build weapons. Unwilling to let Zanarkand be destroyed, Yu Yevon had the surviving citizens turn themselves into fayth and used them to summon an idealized version of Zanarkand at the peak of its power. Yu Yevon gathered a horde of pyreflies and transformed them into an armor for himself, which became the creature, Sin.
Yu Yevon gave the creature a set of simple instructions: destroy any machina or large settlement to stall technological evolution, and retaliate to all hostility with extreme prejudice. Sin's first act was to destroy the real Zanarkand, and as the armies of Bevelle saw the destruction they retreated, bringing the first news of the great monster. Eventually, a religion was created around Yu Yevon's name, his worshipers not realizing they were praying to the one responsible for Spira's cycle of destruction.
According to the fayth used to summon Dream Zanarkand, Yu Yevon is neither good nor evil. His existence only has one purpose: to maintain Dream Zanarkand while protected by his armor, which is why he is drawn to the Final Aeon when his former armor, Sin, has been destroyed.
Shortly before creating Sin, Yu Yevon informed Yunalesca of his plan and gave her a means of calming the creature. Every time a summoner destroys Sin, the aeon used for the Final Summoning is possessed by Yu Yevon and used to rebuild Sin. The period of time Yu Yevon is rebuilding his armor is known as the Calm.
One thousand years later after Sin has been reborn many times over, Yuna and her guardians pierce its armor and defeat Yu Yevon. This temporarily banishes Sin (it later returns in Final Fantasy X -Will-, ending Yuna's-then Calm) and allows the fayth to cease dreaming.
Yu Yevon is the final boss of Final Fantasy X. He is accompanied by two Yu Pagodas. He uses high level magic, but the party is under constant Auto-Life effect provided by the fayth. Yu Yevon lacks magic that disables party's actions in a battle, like Petrify, to counter the Auto-Life.
Behind the scenes
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.
Yu Yevon is represented by the letter A in the Yevon script alphabet. This symbol appears on his true body and in multiple places in Spira, sometimes paired with the letter Z, the symbol of Sin, symbolizing the beginning and the end of things.