i know this is an over talked about topic but never have a read a post that went this into depth so hear me out and tell me what you think

first what i know is that jecht was lost at sea while training for blitzball, also yevon created the fayth at gagazet to preserve zanarkand as it was 1000 years ago. He did this when he knew the war with bevelle had been lost. Yevons first inteintion for sin was a shield to protect him while he summoned dream zanarkand. this is comfirmed while speaking with mika at the highbridge he says that yu yevon crafted the souls of the dead into an unholy armor or sin. When dream zanarkand was summoned yevon transformed to yu yevon and not long after yunalesca defeated sin using the final aeon. sin is reborn yu yevon attached himself to the the final aeon. until yunalesca destroyed sin it was only a shell for yu yevon and when it was destroyed yu yevon merged with zion and sin was reborn as the first sin that is known in modern spira.

pulling from all this we can assume that sin was created to protect dream zanarkand, dream zanarkand is an "aeon" of sorts born from the dreams and memories of the people of zanarkand that were made the fayth at gagazet. so sin is connected with dream zanarkand

my personal belief is that dream zanarkand is a place in spira but is unattainable by the people of spira.

summoner yucon's final aeon much like jecht did when he was sin may have had some control over sin after all sin does not possess a mind of its own it is the mind of yu yevon and the final aeon together. if we can assume this is true then we can guess that the person that choose to be yucon's final aeon was a rightous person like jecht rather then how seymour wanted to be with sin. therfore he may have been looking for a way out this is were this part gets iffy when sin killed yuna's mother braska decided to become a summoner this is a fact not long after the death of yuna's mother jecht appeared in spira and braska sought him out when he heard of his stories about zanarkand this is a fact.

i believe that this was a plan that arose from an accident by yucons final aeon

yunas mother died making braska want to become a summoner when this happened the plan began. but yu yevon was still using sin to protect dream zanarkand. during jechts training out at sea he comes into contact with sin and is warped to spira. ifso facto braska becomes high summoner jecht becomes sin

jecht and auron formulate the final plan and jecht takes him to be tidus mentor before sin is reborn.

tidus' move to besaid was the mark of the begining of their plan. auron hints at this when he asks sin if he was sure. thus begining the game


this is what my theory of sin, dream zanarkand, jecht, and the whole story is basically let me know if you find any flaws and give me your thoughts on what you think