Sin was what Tidus, Auron, Yuna, Wakka, Lulu, Kimahri and Rikku had to fight. It was his dad though, poor sod. Tidus' dad, Jecht. Jecht assumed the personification of Sin all the while, with Tidus thinking he was just some washout blitzball gloryhound that left him.

That wasn't the case! It was tough going but they came to a resolution. Let's not get into Seymour and all his faggotry but let's just say, sin is a big giant whale with advanced magic and durability.