(Man, I really need to play 6 again)
Perhaps Celes turned away as the emperor put more and more faith into his newest general Kefka. The man was clearly insane; his actions even drove General Leo to defect (though it didn't work out so well for Leo) after that fiasco with the Espers. Seeing something you believe in fall to the madness of a charismatic lunatic can be pretty jarring, to the point where death seems the preferable option.

I don't know, though. It sort of feels like the plot started as a story about a magical amnesiac but Terra didn't have any romantic prospects, so they redirected to another lead. I mean romantic prospects in both senses: an interpersonal relationship with someone who literally knows nothing about the world or her past is inappropriate and the tension of knowing nothing is shattered as soon as the character learns her past; there's just not a lot of room there to be dynamic as a protagonist.
Conversely, Celes presented several avenues of literary growth; she walked the road of seeing the corruption behind something she believed in, suffered the catastrophic grief of knowing that her actions directly lead to the destruction of the world, and carried the weight of knowing the man she had feelings for was using her as a surrogate for his dead fiance. She's an incredibly deep character and perhaps the writers felt that supplying more details of her past before the defection would rob the audience of making their own leaps. It's a parabolic correlation, where too few or too many details generate disinterest in the audience.



Changing the subject, in the Sega Genesis version of the Jurassic Park game you can play as a raptor, but the raptor is specifically hunting Doctor Grant. There are literally DOZENS of guards in the game (which, by the way, doesn't make sense because the island was evacuated in anticipation of the incoming tropical storm) but none of them are good enough to eat. No, it has to be Doctor Grant. ...Why?