Really, Ultimecia? Her presence is "there" through most of the game, and when they reveal her it isn't so out of left field. It's definitely nothing like IX.
Anyway, I think this has really strayed quite a bit. I was just interested in getting a more detailed view on why Leyvuir saw the story and characters in FFXIII as bad. It also baffled me drawing the comparison to XII's story (which, IMO, is among the worst) but to each his own of course.