This could explain the way Sora looks in the trailer, too--perhaps he's been asleep because he's slowly fusing with Yozora into the same person because the universe can't handle two of the same person within one universe and we don't realize that until the end of the prologue or something like that.
I'm starting to believe that this is an AU version of SRK at this point. Nameless Star is AU Kairi, the heart within Yozora that's not his own is AU Riku, and the person Nameless Star is in love with whose heart was cast out is AU Sora*. After all, Riku had his heart cast out by Ansem SOD in this universe. Perhaps in another universe, Riku is the one who casts out somebody's heart, in this case AU Riku may have cast out AU Sora's heart and taken his body in order to have the power of the Keyblade when AU Sora is revealed to be the true master of it (I'm thinking of whatever the AU version would be of when Sora and Riku fight in Hollow Bastion and the Keyblade leaves Riku and returns to Sora). Or it could be exactly the same, with AU Ansem SOD in AU Riku's body (having cast AU Riku's heart out) merging with AU Sora's body to have his power for the Keyblade. That could explain why AU Kairi is dead, since Ansem SOD planned to get rid of Kairi once her purpose was over in KH1 until Riku stopped him.
* And how fitting that Sora would have been drawn to Kairi's heart somehow when he ended up in the Final World--the only one available where he was being AU Kairi rather than the Kairi from his own universe. Perhaps this is also why, when Sora returns back to the Keyblade Graveyard, Nomura chose Kairi to be the one who returned him. Sort of a book-end going from AU Kairi back to the real Kairi.
EDIT: I'm re-reading Nameless Star's dialogue right now and it sounds a great deal like things an AU Kairi would say. She only exists where she is because she pines for another (Kairi's entire character revolving around Sora this whole series) and she'll wait for him to come find her (her whole story being one of waiting for Sora)... I think most of us just assumed Nameless Star must be whoever Saix was talking to Axel about because she was introduced in the same game that plot was, but maybe the two aren't connected to one another at all. It makes sense that the Fairy Godmother says Nameless Star's heart would "certainly be stolen" if it went back to its universe in that case, because she is a PoH. Perhaps AU Kairi's heart being in the Final World is the only reason Sora could come to save Kairi at the end of MoM (despite being unable to speak), since their hearts are connected with him becoming one with Yozora.
Also, look at this:
Star: So I choose to wait here, where he can find me.
Sora: Good. You have to believe.
Star: Believe? Oh, you mean know in my heart he will return? Without any proof?
Sora: Exactly, BELIEVE. I thought it was all over for me, but a friend of mine looked me in the eye and said, "You don't believe that."
Versus this when he returns:
Kairi: You see? I had no doubts that you'd pull through.
Sora: The light in the darkness. It was you. You're the one who kept me from fading away.
Kairi: All I did was believe that you wouldn't.
I have some things to add to Riku being AU Yozora thing.
: I accidently wandered into this place, and went through some trials. Then, was told to "save Sora."
This is basically Riku's whole plot in Dream Drop Distance. Firstly, Riku accidently ended up in Traverse Town. He also accidently ends up in Sora's Dream and if you believe my theory that Yozora has the power of waking, Yozora is also in Sora's dream when he says "I accidently wandered into this place". Secondly, Riku goes through the mark of Mastery of which Yen sid says about Sora and Riku's Mark of Master is "more trial
than test". Lastly, Yen Sid tells Riku to wake Sora at the end of DDD, which is very similar to "save Sora".
There are a couple more things that make him similar to Riku. Obviously, there's his appearance. He looks like RIku. Sora, Donald, and Goofy all think this when they talk about Yozora's appearance after they see the Verum Rex box art in Toy Box. If Sora loses to Yozora he says:, "Sorry, but I don't lose", which is the kind of arrogance we see from Riku in KH1.
Alternatively, Yozora is like Roxas in many ways to. Firslty he was introduced in a secret ending. Secondly, his name is a play on "Sora" (Sora means "sky", Yozora means "night sky") just like Roxas' name in a play on "Sora" (Roxas is an anagram of Sora with and "X" inserted in). Thirdly, Yozora's earinng is the same symbol that acts as Roxas' zipper in Roxas' street clothes. Fourthly, during the Roxas vs Sora fight, Roxas withholds Sora's keeblade briefly just like Yozora. Fifthly, that fight takes place on Sora's station of awakening, just like Yozora's boss fight.
What's interesting to note is that Yozora vs. Sora also parallels Roxas vs Xion. Xion shows up to the clocktower with Sora's face; Sora shows up with Sora's name. Xion forces the fight to happen, "this puppet will have to play her part", she says; Yozora forces the fight to happen, "If you are who you say, and it was fate that brought us here, then... my paath is clear", he says. When Xion dies, she crystalizes; If Yozora wins, Sora gets crystaliezed.
the person Nameless Star is in love with whose heart was cast out is AU Sora
I think you might be reading something that's not there as there's no evidence that the nameless star "loves" the person she's talking about.
I'm re-reading Nameless Star's dialogue right now and it sounds a great deal like things an AU Kairi would say
I actually think, the Nameless Star's dialogue sounds more like Xion than Kairi
: So whose heart are you?
: No one's anymore. They took away my name-- everything about me.
Everyone forgetting Xion very much sounds like "they took away my name-- everything aobut me"
: But, were he to regain his old self again, he would be distressed by my absence.
This sounds like what happened right after Xion dies in 358/2 days. Roxas gets distressed and immediately tries to save Xion.
: So, uh...who did this to you? Heartless steal hearts, so...a nobody?
The hesitation in the Nameless Star's response is telling. This is the kind of response that I think that Xion would give because, what happenedn to her? She was killed by Roxas, a nobody; but he's a nobody with a heart which makes him a somebody. And considering the conversations they have about not having hearts and being nobodies in 358/2 Days, Xion would want to make this distinction.
Fairy God Mother talking about the Nameless Star
: If she goes back to her world, her heart will certainly be stolen.
You could imagine an AU version of Xion not willing to return to Sora, you'd have Riku, Mickey, and Diz having to forcefully do it. In other words, they'd have to "steal" her heart.
This could explain the way Sora looks in the trailer, too--perhaps he's been asleep because he's slowly fusing with Yozora
I think Nomura said during an interview that Sora looks like that because he's in Quadratum and that, in other worlds, He will have a more KH art style again. Donald and Goofy having that KH art style in the trailer was supposed to convey that.
I do think that Sora and Yozora will fuse together, though not slowly. It would be quick like Xion or Roxas. This goes hand in hand with my theory that I posted above, but I'll copy paste it here:
I believe that Yozora has had his heart transferred from his original body into this Riku lookalike body that we meet. The nameless star says about (presumably) Yozora, or at least the Riku lookalike body, that "he's been changed beyond recognition--his heart replaced with another's". That seems pretty clear about what that means. It's supported by Yozora saying, "This isn't what I really look like". The original owner of the Riku lookalike had his heart removed and somehow, Yozora's heart ended up inside.
We actually can make a good guess as to what Yozora's original body looked like. In Toy Box, Rex and Hamm recognize Sora as Yozora. This is odd because Sora doesn't look like Yozora. Sora mentions this and Donald agrees, so why? Rex and Hamm have played Verum Rex which is a game about Yozora, so if Rex and Hamm think Sora is Yozora, it stands to reason that they saw Yozora's original body when playing Verum Rex and it looked like Sora.
I think that Yozora's original body not only looked like Sora, but was Sora himself. I called Yozora a "heart" earlier, but that was because of a lack of a better term as I don't quite know exactly what he is based on what I'm about to say. I think that Yozora is the power that Sora lost when Sora briefly fell to darkness in DDD. To explain: Yozora is one of the most difficult boss fights in all of Kingdom hearts and if his skill and strength come from Sora, this difficulty makes sense as, by the end of the Mark of Mastery DDD, Sora would've been a keyblade master if he didn't lose his power. This is admittedly a weak argument, but what's not weak is what I'm about to say next. Yozora has the power of waking. Why do I believe that Yozora has the power of waking? He literally and blatantly shows us he has it when he uses it on Sora. To explain: Young Xehanort describes the power of waking as the ability to traverse hearts to reach worlds. This is consistent with how we see it used by Riku at the end of DDD to wake Sora. If you remember, he dives down onto Sora's station of awakening (a heart) and then is transported into Sora's dream version of destiny islands (a world). What does Yozora do? he points his crossbow at Sora where then Sora's station of awakening (a heart) appears from underneath them, and then Quadratum (a world) renders in around them. It's clear, Yozora has the power of waking.. Lastly, Sora means "sky" and Yozora means "night sky", what would be more appropriate for a character born from Sora when he fell to darkness than "night sky"? It's not very subtle when you think about it.
Just as an aside, you might be asking why they end up in Quadratum if the power of waking takes them into Sora's dream. Sora has never been to Quadratum, so how could it possibly be Sora's dream version of Quadratum? Simple, Sora has been to Quadratum. In Toy Box, Young Xehanort throws Sora into the Verum Rex videogame where he fights a bunch of Gigas in Quadratum. We know that the power of waking took Sora and Yozora into Sora;s dream version of videogame Quadratum because of how it "renders" in rather than just appearing like Sora's station of awakening does.
But If you don't want to read it, the conclusion is that: Yozora is the power that Sora lost in DDD
. It would make sense that they would recombine into a single person again if that were the case.