The way you use language here is good, but there's some serious plot holes. For one thing, Seifer already exists in the KH universe, he's a fifteen year old punk in Twilight Town. You didn't give the reader a chance to expect a change to canon like that; it feels like you're just ignoring the game. Also, I don't know why it was suddenly okay that Seifer's been killing people and taking their hearts. The problem that hooked the characters into the story to begin with is suddenly ignored once everything's set up. That means that the initial conflict you set up never got resolved: bad storytelling. In the future, I'd suggest putting more thought and effort into the logic and flow of your plots.
Author's response
I'm sure you've come across stories where once the basic premise has been accepted, the reader is asked to go along with the writer's interpretation. As for the reason behind Seifer's actions... if you didn't get that then it's natural that the story failed in your opinion.
Anyway, I don't mind the criticism or your dislike of the fic. I simply thought your long review deserved a reply.