Seymour is sorta the villian in the middle of the game and towards the end. I guess you can call him a side villian or somthing
Seymour's parents were the first Guado and Human to marry and have a child so Seymour is the first human/guido. The Guido look up to Seymour as the link that will connect humans and guidos together.
His mother wanted Seymour to have the power to protect himself so she became an aion (anima) for Seymour. But after a while he wanted more power and became upsessed with it. After a long search for his new power he finally sees it... Sin.