Final Fantasy X
Voice Actor: N/A
Sin gained his name from the people of Spira. They were convinced that Sin was created because they had to repent for their sins which lead to the great Machina War, a war which decimated their world a thousand years prior to the start of Final Fantasy X.
Its purpose is to destroy all technology and dwellings before they become big enough to cause a considerable threat, and it has done this job for a long time due to an ongoing cycle which allows it to remain strong. This cycle is the pilgrimage of a Summoner.
Maintaining Its Continued Existence
In truth, Sin is a shield which houses Yu Yevon, its creator. He uses it as his armour to protect him and uses his daughter Yunalesca, to ensure that he remains safe, by revitalising Sin. Everytime the Final Aeon is summoned and defeats Sin, Yu Yevon then possesses the Final Aeon and it in turn becomes the new Sin.
When Sin is defeated, it causes a Calm, but the lives of the Summoner and Guardian picked to be the Final Aeon are lost during the process. During this time, Sin grows until it is once again strong enough to carry out its main task.
Plotting Its Permanent Defeat
After finding out the truth of Sin's existence, and how it will always come back, Yuna decides to try and break the cycle. They defeat Yunalesca (thus rendering the Final Aeon an impossibility) and manage to get inside of Sin, where they face the previous Final Aeon.
After defeating Jecht (the Final Aeon), Yu Yevon is released from his control and tries to attach himself to all of Yuna's Aeons. However, when he runs out of Aeons to possess, he is defeated and Sin is permanently destroyed.