According to the theory:
There will be an Ultimecia in the future. Her existence, persecution, and defeat are inevitable. You can’t change the past, and Ultimecia’s death in the past created the present and future. Now what if that Ultimecia was not a random person yet to be born, but one of the fighters who would perpetuate her own tragic destiny?
Who is Ultimecia? Is Rinoa Ultimecia? Fans have made a ton of connections between the two, due to the former becoming a Sorceress.
How about Selphie? She has some of the most powerful magic in the game.
What about…all of them? Every female becomes Ultimecia, given time compression. Since all women have high magical stats, anyone could grow to become Ultimecia, given enough time.
Final Fantasy VIII exists within a partial stable time-loop. Since Ultimecia travels back into time, and Squall warns Edea of SeeD, and the cycle resets. However, the time loop resets with minor differences; some women become Ultimecia.
In this sense, Ultimecia isn’t just one person. She’s a title that many of the women in Final Fantasy VIII take in the future. And Ultimecia takes on the powers of every single woman that becomes her. This explains her powers to fling her consciousness back into time. This is Ellone’s power, and thus she becomes Ultimecia. In this sense, Squall has to defeat his sister.
You can’t change the past. I just found that out.
But it didn’t work. I can no longer make it back to that moment… And also… I can only send you inside someone I’ve met before. I’m sorry, Squall. I’m about to disconnect. I’ll try again to speak to you this way.
While no one changes the past, somehow Ultimecia can possess Edea and Rinoa to take actions they wouldn’t. She can also only possess those who will eventually become her. In this sense, Ultimecia becomes Rinoa in the original Final Fantasy VIII story we play. But eventually, so does Selphie and Quistis. Of course, they have to be around the same age, for her Ultimecia to fling her consciousness around the same time.