According to the Secret Reports' original video, the character Brain in Kingdom Hearts Union Cross displays consistently inconsistent behavior especially in regards to the Book of Prophecies, which is currently one of the most important artifacts in the lore of the Kingdom Hearts Series. This theory proposes that the reason Brain seems to contradict himself is that he has an imposter that is meddling with Data Daybreak Town for some mysterious purpose.

Most recently, this theory has gotten more attention again from Sonny Novus's video discussing the status of Brain after the events of the final KHUX update, naming Luxu as a primary suspect due to the...everything else going on with Brain and Luxu in the time period between KHUX and Kingdom Hearts Missing Link.

I think that if this theory is correct, there's currently only two candidates that stick out for me that would make sense and thats Master Ava and Luxu.

For Ava, she's one of a very very small pool of Kingdom Hearts characters we know to have a strong aptitude for Illusion magic which would allow her to easily pose as another character and obviously has a vested interest in the Dandelion's continued survival and has questioned the Master of Master's plans and motives in the past. Ava in general is also just currently a big question mark lore wise as she's seemingly been out of the picture (and more specifically, Luxu's watch) since the original Keyblade War. I like Ava as an impersonator candidate cause it would at least give us some baseline for where she went after the war as well as following up on her relationship with Brain, even if it's in a more manipulative light.

Luxu is probably the more perplexing option, as this assumption raises a lot of strange questions regarding his actual appearance considering so far we've only seem him with his hood off when he is already occupying a vessel. Kingdom Hearts is a series that takes identity very seriously, so Luxu's original form physically resembling Brain would impose some very interesting implications regarding the two's origins and symbolic connections to eachother. I have two thoughts on this. 1. Luxu possessed Brain's data copy and 2. Luxu or Brain are eachother's real-world/data world counterpart. I have very little evidence for both of these especially considering their supposed age discrepancy. That said, I can't help but think that Brain's data copy has something to do with this as Brain is commonly associated with computer viruses and other technical anomalies.

Also this has nothing to really do with the theory but this is weird so I felt like I should mention it here but Xigbar (Luxu's current and most recognizable vessel) and Eraqus (Brain's Grandson) have a very specific parallel to eachother in that they both gained their facial scars from a Dark-aspected attack from Terra and Master Xehanort respectively as seen in Birth By Sleep. I don't know what this means but its still super strange in the context of Luxu and Brain's larger history especially since Eraqus seemingly has some of Brain's memories??? For some reason???

So in Kingdom Hearts 3 we first learn about Subject X and that Lea and Isa discovered a girl being kept and studied by Ansem the Wise's Apprentices in Radiant Garden and she was the deciding factor for them to join the Apprentices as she eventually went missing and they wanted to try and find out what happened to her. According to the KH3 secret reports, we learn a bit more about her defining traits. According to Apprentice Xehanort, the girl was found in the central square of Radiant Garden and was seemingly around 15 year olds. Subject X displayed a profound case of amnesia, aside from a few vague memories regarding her companions and most interestingly, knew the phrase: "May your heart be your guiding key" which in universe is a phrase only known by Keyblade Wielders, specifically those with knowledge of the Age of Fairytales.

Subject X also has one report penned by herself and is currently the best lead we have for where she went, as she stated a man wearing an eyepatch was the one who released her from her confinement. In this report, Subject X also specifies that her recollection begins and ends with "four friends and a key" which by process of elimination, should already give us a pretty clear picture of who she is and why Braig/Xigbar was the one to release her.

So like. This is Skuld, right? Like. It obviously has to be Skuld.

Skuld is the only member of the Dandelion Union Leaders we don't see given a clear destination in the KHUX finale and her memories matches up perfectly with what little Subject X can recall. It would also make sense for Luxu (as Braig/Xigbar) to be the one to free her from Radiant Garden as he has consistently been involved with ensuring the Dandelion's, specifically the Union Leaders, survival even at this point in time as he was the one to recruit Marluxia (AKA Lauriam) to Organization XIV presumably a few years later.

If all of that wasn't enough, Nomura confirmed in 2019 that Subject X's true identity was that of a character who already appeared in the series at that point and various extra material has Saix specifically describing Subject X as having black hair so...why why why have we gone this long without getting a confirmation on this?

I really want to believe that all my doubts on this are just from the mystery being dragged out for 5 years now cause personally, I feel like any other option would narratively be a great disservice to Skuld. I don't like the idea of Skuld ultimately just being related to Xehanort's mom (who yes, does look a lot Skuld for some reason) and thus just being relegated to a sidelined maternal role. I also don't like the idea of Ava being Subject X as to me that would ruin her dynamic with Luxu as peers who disagree on what needs to be done to ensure the Dandelion's legacy lives on as ultimately it would mean that she would need to be bailed out by him without knowing it was even him.

This mystery has honestly become very frustrating to me as since KH3, there was a chance in KHUX and KH Dark Road to finally put all the speculation to rest but it's just kept going and now I have to second guess myself even though none of the alternatives feel as interesting and satisfying to me as the seemingly obvious conclusion. KH has engaged in meta-commentary in the past with characters such as the Master of Masters calling out people who get hung up on their assumptions so I don't want to be one of those people but also like. I really like Skuld as a character and I feel like she's so overlooked even by fans of KHUX and it'd just feel like a waste of potential to essentially just leave her as a relic of KHUX's past. I'm still like 90% certain Skuld is Subject X, but man. I can't lie and say that the anticipation isn't getting to me a little.