I see your ‘Ultimecia is Rinoa’ theory and raise you ‘Ultimecia is the Great Hyne.’
Explain.
I’m glad you asked/demanded on pain of death.
Okay, so the Great Hyne is purportedly the god who empowered sorceresses, and you can find about five different variations of the myth about him, all of which are contradictory – and in fiction, ‘multiple contradictory myths’ is usually shorthand for ‘this is semi-true, but has been distorted.’
Of the features consistent across the myths, we know:
Hyne has ‘two halves,’ and he gives half of his body to humanity (with myths differing on whether the sorceress power is the half he gives, or the half he keeps), while the other is never found.
People go to war with Hyne, and Hyne kills or abducts children.
Hyne is the source of every sorceress’ power.
Then we have Ultimecia, who becomes a goddess of sorts when she performs time compression. When she does, she looks like this:
She has two distinct halves to her body: The powerful upper half, and the bound lower half. The upper half also forms part of the backdrop in the fight, showing stars and planets, indicative of Ultimecia achieving a deified nature.
When Ultimecia is killed in this form, she’s actually still alive – her human form (which is her lower half here) escapes and gives Edea her power, because a sorceress can’t die until they’ve passed on their power.
We’ll get to that later, but first: Hyne goes to war with humans, and kills children (either as the instigating incident in the war or as part of it). Ultimecia was, we know, persecuted for being a sorceress (in a stable time loop where she attempts to alter the past to become a god in order to escape that persecution, but her actions in possessing other sorceresses creates their bad reputation which leads to her being persecuted), but we also know that Ultimecia specifically fights SeeD: Outside her castle are the bodies of dozens of White SeeDs, and SeeD are all child soldiers, at least when they’re starting out. So Ultimecia does kill children.
Back to her giving Edea her power: Notably, Ultimecia is apparently the only sorceress of her era, and one recipient can gain multiple sorceress’ power (because Rinoa does just that), so it’s likely that in the far future, all of the sorceress lineages converge on Ultimecia somehow.
Edea, however, is just a regular sorceress with a single sorceress’ power, not the recipient of dozens or hundreds of sorceress’ power.
Moreover, the Junction Machine Ellone that Ultimecia has apparently been using to possess sorceresses from across history can, if it functions like Ellone’s powers do, only be used to transplant someone’s mind through time to the body of someone they have a personal connection to. Ultimecia has a connection to Edea through giving Edea her power, but she shouldn’t have one to Adel, who she also possesses.
Instead, it seems plausible that Ultimecia is the originator of all sorceress lineages: After her defeat, she’s fragmented through time into as many copies as she has sorceress’ powers, and appears to the sorceress at the start of each lineage, granting them one sorceress power each. This in turn makes another stable time loop where they pass on their powers until Ultimecia becomes the eventual recipient of all of them, is defeated, and is fragmented through time to start the loop again.
This would also explain why there’s apparently an ancient legend about a ‘Legendary SeeD’ defeating her (and that legend sets Ultimecia up to hate and fear the SeeDs), despite the fact that only Squall’s party was present for it – Ultimecia tells a woman the story as she gives her the sorceress power, and they pass it on, while others to whom she gives the power pass on the story of the Great Hyne. Essentially, a third stable time loop where the prophecy drives Ultimecia to actions that ultimately result in her telling people about it, which then becomes a prophecy.
If this scene isn’t in the remake I’m getting on a flight to Japan, bringing an original Playstation, seducing my way past Square Enix secruity, kicking Nomura’s office door down and throwing the Playstation right in his stomach