Sin is Jecht. I am going to throw that out there right away, in case anyone has not played Final Fantasy X. However, Sin is more than just Jecht; Sin is an amalgamation of Jecht, an ancient summoner, and a horrifying monster-weapon. As a character and antagonist, Sin is an oddity. It is a creature without a consciousness, compelled to destroy all civilization. However, Sin is made up of two main entities: Yu-Yevon the summoner and an Aeon that surrounds Yu-Yevon. Sin is a summoned creature created by Yu-Yevon during Bevelle’s invasion of Zanarkand. He commanded Sin to destroy Bevelle in retribution for Bevelle’s attack on Zanarkand. Sin’s body is created from the remains of vanquished Aeons. Sin is part of a cycle in which it wreaks havoc on Spira before being destroyed by a summoner whose guardian becomes an Aeon. After Sin is destroyed, there is a time of calm in Spira, but Yu-Yevon creates another shell for Sin from the Aeon that defeated Sin – and so it lives on.
Generally, Sin is a mindless creature controlled by Yu-Yevon’s whims, though it is unclear as to whether Yu-Yevon is capable of rational thought. After spending many years as a part of his monster Sin, Yu-Yevon no longer possesses a physical human form. He is a speechless bug-like fiend creature that vaguely resembles a metroid. He destroys towns, villages, and cities as Sin but when confronted does not seem to have any higher cognitive function left. He fights the protagonists mindlessly, only able to discern when he has been harmed and then respond by magically healing himself. He is unable to comprehend when he is affected by magical status effects such as Zombie, indicating that he cannot comprehend magic beyond what he is capable of. For all intents and purposes, Yu-Yevon is a self-engineered destruction machine, unable to reason or think. Because he cannot be reasoned with, he is very dangerous as Sin, but because he is almost nonsentient he is not very dangerous in his fiendlike form. With an almost indestructible Aeon shell, Sin has struck fear into the hearts of Spiran residents, whose religion transformed Sin from a destructive monster into a dark god of retribution for the sins of the world.
However, during Final Fantasy X, something strange happens with Sin. Tidus and Jecht are created by the same spirits that grant summoners in Spira the power to call Aeons, for the purpose of destroying Sin for good. Jecht is a strong-willed individual that becomes the guardian and later Final Aeon of the summoner Braska. As with other Final Aeons, when Jecht defeats Sin, Yu-Yevon creates a new shell for Sin using Jecht. Strangely, Jecht is not completely destroyed by this. Instead, for much of the game, Jecht has a small amount of control over Sin, indicating that perhaps his will is stronger than that of the now almost mindless Yu-Yevon. He is able to allow Tidus and Auron to ride on Sin from the false Zanarkand to the real Spira, and on a few occasions he even forces Sin to flee and spare the life of Tidus. In a way he is even able to speak with Tidus for a short time, conveying that Tidus needs to destroy Sin permanently in order to create a lasting calm for Spira. The protagonists of the story are able to calm Sin when they realize that Jecht is calmed by the Hymn of the Faith, a religious song in Spira, and that when Jecht is calm, Sin is calm. When the party enters Sin and finds their way to Jecht, Tidus and Jecht proceed have what is supposed to be a heartfelt discussion about their lives. Jecht is harsh and unfatherly both in the beginning of the game in Tidus’ memories, and in the end inside Sin. Jecht reveals to Tidus that Sin is taking control, and that soon he will become part of Sin completely, transforming into the same mindless beast that has destroyed much of Spira for many years. He states that he will be unable to hold back in a fight against the group, but encourages them to destroy him so that they can finally face Yu-Yevon. When Tidus tells Jecht that he hates him, it does not seem to upset him overmuch. Though it is revealed later in the game that he loved Tidus and did much to help him, he is still very harsh to Tidus, demeaning him for crying both as a child and an adult. He does not allow much outward emotion to show through his confident demeanor.
When the party finally confronts Yu-Yevon, what they find is something pathetic. In a last-ditch effort to defeat the party, Yu-Yevon assumes control of the party’s Aeons, unwilling to fight the party himself. It is entirely likely that Yu-Yevon is aware that he cannot win against the party. After the Aeons are defeated, the battle with Yu-Yevon himself is awful. Yu-Yevon is unable to understand that the Gravija spell cannot kill a person without some other form of attack because it deals damage based on their current status, and he continues to heal himself when hit even when zombified, which only quickens his demise. I believe that all of this overwhelmingly points to two possible conclusions. Either Yu-Yevon is already dead and the bug-creature that controls Sin is a fiend that was made when Yu-Yevon died and was unsent, or the years of isolation from people inside Sin has driven Yu-Yevon to insanity. I think that the more likely explanation is that the unsent Yu-Yevon slowly turned into a fiend over time, and he now no longer has his sanity or cognitive ability. I believe that the fact that Yuna performs a Sending ritual at the end of the battle with Yu-Yevon and that Yu-Yevon never reappears is evidence supporting the theory that Yu-Yevon is actually a fiend.
Whatever the case, the mindless Sin does not provoke as much thought from players as some of the other villains in the video game multiverse. Sin is simple and evil, and though Yu-Yevon’s original reason for creating Sin was a valid one, no one can justify the actions of Sin in the world of Spira. Spira is better off without him.