The heart of Final Fantasy VIII is the love story between Squall and Rinoa. Most of the development for other characters and party members featured in the title falls to the wayside so their romance can shine. That said, there's been a popular theory that Squall and Rinoa's love does not have a happy ending, positioning that Rinoa will become the title's main villain, Ultimecia.
Layering themes centered around moving forward, the title rejects ideas of living in the past regardless of the future's uncertainty. These themes factor some credibility to the theory when examined with some evidence presented in-game.
Fallen Princess?
Before diving into the theory, some primer makes understanding the evidence a bit more digestible. In Final Fantasy VIII, a small group of women known as Sorceresses can wield near-incomprehensible magic abilities and transcend mortality. One is not born a Sorceress, but rather, it's passed down from one to another. Not all Sorceresses are evil, but they're still feared by people and often forced to the edges of society.
Sorceresses usually take on a male guardian known as a Knight who swears to keep the Sorceress safe regardless of personal cost. Both Sorceresses and Knights often utilize summoned entities called Guardian Forces in addition to their own strength. Guardian Forces are magically summoned beings that wield and can imbue impressive powers to their summoners, but they gradually cause memory loss to those who frequently call upon their might. Both Squall and Rinoa use Guardian Forces throughout FFVIII.
Twisted Sorceress
The theory suggesting Rinoa becomes Ultimecia hinges on a few different pieces of evidence players noticed in FFVIII. Ultimecia herself is a Sorceress from an undisclosed time in the future with aspirations to compress time into a singularity. Her backstory and motivations beyond this goal are left vague, which is uncharacteristic of Final Fantasy villains. At about halfway through the game, Rinoa becomes a Sorceress and takes Squall as her Knight.
Rinoa often laments about the uncertainty of her future as a Sorceress, saying that she'd preferably maintain the life she has in the present with Squall. She even goes as far as to contemplate her demise at her friends' hands, "I guess it's okay if it's you, Squall. No one else." Rinoa's wishes to stay in the present where she's happy with Squall sound eerily similar to Ultimecia's plans to compress time into a single point. This creates the basis for the theory; Rinoa outlives her friends and Squall with her Sorceress longevity, slowly descending into madness from grief and adverse mental effects associated with using Guardian Forces over long periods of time.
Another reinforcing element of the theory is Ultimecia's Guardian Force, Griever. Naming is significant to Final Fantasy creators and always bears a more profound meaning than just the character's name. For example, Rinoa's Dog is named Angelo and references her angelic nature and demeanor. Griever resembles a lion, an iconography that is tied to Squall in both his name and the ring he wears. The theory suggests the Guardian Force is a manifestation of Ultimecia's grief following Squall's death and thus takes a form connected with her Knight.
Supporting this theory is a major quest in the game focusing on the ring. In a moment where Squall speaks to Rinoa about his idea of strength, he gifts her the ring, which he says is a creature called Griever (the player has the opportunity to rename the ring, but Ultimecia's summon will retain this name). Squall only spoke about this to Rinoa and no one else. When her Knight fell, did Rinoa create a new guardian in the form of her love's symbol of strength?
Seeds Of The Future
The theory's biggest problem is that it would go against Square Enix's general desire to ensure titles at the time left players with a cheerful ending. Even though Square titles often deal with mature themes like death and war, they'd almost always end on an upbeat note for the characters involved. There's also the matter of Ultimecia being able to read Squall's mind and access his memories without reacting to what would surely stir up some emotions. She'd see memories of Rinoa and Squall, and if the theory were correct, memories that'd actually be herself and Squall.
Perhaps most damning for this theory, though, is that the game's director, Yoshinori Kitase, has gone on record debunking the theory. He's stated, "No, that is not true, I don't think I'll incorporate that even if we do remake the game. But that being said, both Rinoa and Ultimecia are witches, so in that sense they are similar, but they're not the same person."
Though the title's director officially deflated the theory, many connections are difficult to explain away. It's certainly possible the theory points to a direction Square initially intended for FFVIII but ultimately decided against. If that is the case, it's unlikely Square will ever admit to it, and Rinoa and Ultimecia's connection will remain uncorroborated.