Rinoa is Ultemicia

[Bloodline666]

Like Skyblade pointed out, Ultimecia's attempts to kill herself and Squall etc. are usually explained away by assuming that Ultimecia, despite being Rinoa, was so far removed from Rinoa mentally that they had essentially different identities and Ultimecia would have no recollection of her past. Of course this makes one wonder why bother with the theory to begin with, but there you are.

The most damning evidence against the theory is the complete lack of good evidence in favour it, both in the game and in the Ultimania.

That's not the most damning evidence against the R = U theory; the most damning evidence against the theory is, in fact, explictly stated in Ultimania, itself. According to that very guide, Rinoa will die before Ultimecia is born (if that doesn't directly dispel the R = U myth, then I don't know what does). As mentioned earlier, Ultimania states that a Sorceress' lifespan is equivalent to that of any other human, and Ultimecia lives too far into the future for any of the other in-game characters (Rinoa included) to exist in that time period (even the in-game script, itself, confirms that Ultimecia lives "generations into the future"). How many Sorceresses have inherited Rinoa's powers between Rinoa and Ultimecia is not explicitly stated, but since Ultimania explicitly stated that Rinoa will die before Ultimecia is born, it is also impossible for Ultimecia to have directly been the successor to Rinoa. So common sense tells me that there has been at least one Sorceress between the death of Rinoa and the birth of Ultimecia. It is very possible that the Sorceress who inherited Rinoa's powers may have passed her powers to Ultimecia, but that possibility is still open to speculation. Nonetheless, that is the closest possible link between Rinoa and Ultimecia. It is also possible for Rinoa to pass on her Sorceress powers to someone else without dying (as shown by the fact that Rinoa, herself, inherited her powers from Edea, who was still alive after the fact, and is still alive at the end of the game), but that is subject to a completely different debate.

One could argue that Rinoa had been sealed in the same manner that Adel was (as was the case for Rinoa, herself, for a very brief period during the game) for decades, or even centuries, but I don't see how that is possible, given the fact that only Esthar has that technology, and the fact that there are politics into play that prevent that from happening, as Esthar's President at the time of the game is not only widely speculated to be the father of her Knight, but could also end up being her father-in-law at any point post-game.