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And a complete personality switch can be used, but normally, the writer will build up this personality change without an event. Do you think any half-decent writer would write a story with a sunny, bubbly, optimistic character, who lives in the moment, and then, without any warning or build-up, turn her into the complete opposite: depressed, pessimistic, so afraid of losing somthing that she's to scared to live, a loner.
A popular 'theory' is that she turned bitter after losing Squall, and perhaps forgot her past because of GF usage. According to Edea, having a knight helps to keep a sorceress honest and good or something. But except for Ultimecia being associated with the GF Griever, there's nothing that even remotely supports these hypotheses about Ultimecia.