SPOILER!!:
Actually, the Jump Festa trailer reveals the scene where the Emperor brings Jecht before Chaos, asking him to "pour his power into him." It basically means before the end of the game, Jecht will switch sides.


I think the real mystery here is why Tidus is not on Cosmos' side this time. He wasn't in the opening CG, except when he headbutted Jecht, and so far,, had been completely under the radar.