Before I start another Yozora theory thread I want to link to some of my previous bits and pieces of theories about him because this theory is sort of building on those: link
Ever since the the secret episode dropped, we've been scratching our heads trying to figure out why Yozora acts in such seemingly contradictory ways- particularly how in one moment he can treat Sora harshly, like a complete stranger he has no sympathy for, and the next moment he's placing a tender hand on the crystallized Sora, making an oath to save him as if Sora is someone he cares deeply about. This theory is offering a potential explanation for this behavior.
Is Yozora just pretending to act like Sora is a stranger to him, just a means to an end? No, I don't believe so. When first meeting Sora he truly shows no sign of recognition and is genuinely surprised when Sora gives his name. A name he's heard but didn't know who it belonged to.
Here's what we know: After a series of trials, in a world he accidentally wandered into, he was told by *someone* to save Sora- but was apparently given no description of this Sora. Yozora questions why Sora is using that name, but it's unclear if this is because he know a Sora that doesn't look like our Sora, or if he doesn't know of any Sora personally, but for some reason expected Sora to look different. It's apparantly also vague in the jp too/can be interpreted either way, so there are no clear answers there.
He also tells Sora that "this isn't what I really look like", so he's surprised Sora can recognize him by his name. There's multiple interpretations to this but for the moment we will assume that he means that he isn't supposed to look like Riku.
From the nameless star, presumably the girl Yozora is trying to save in the Verum Rex trailer, we learn that Yozora's heart has been "replaced by another's", and he has forgotten about her as a result. I believe this heart to possibly be Sora's- or rather, pieces of Sora's.
If you read the previous linked theories, these pieces, broken off by grief, contain Sora's lost memories of Riku- hence Yozora's appearance. But this doesn't mean Yozora has conscious access to these memories. Similar to Xion, Yozora might experience bits and pieces but without context, and without his own lost memories, he has no way of parsing them. He may not even realize they aren't his own... If they were to weave in with his own memories, how could he tell the difference?
Hold on...haven't we seen something similar before? Oh right. CoM.
In Chain of Memories, Namine created false memories (built from existing memories) that made Sora forget almost all of his friends and believe that Namine, a complete stranger, was a precious person to him that he had promised to protect and save. Sound familiar?
What I'm proposing is that Sora -unknowingly- is analogous to Namine here, and Yozora is in a similar position Sora was in CoM. It's not a 1:1 comparison because I think there's a mix of concepts being used with Yozora, but I think this is a primary one.
So, here's my theoretical scenario: Yozora wanders into the not-real Shibuya, having lost his memories of nameless star girl. Whether this is because this is his "price" in something like TWEWY's reaper's game, or because of the influence of Sora's heart, I can't say- But consider that the idea that Yozora knows he is missing memories of someone important to him. He just doesn't know who it is. Then, after undergoing some trials, he meets a mysterious stranger (I'm going to guess MoM).
This stranger tells him that he knows what he's looking for. Sora. Save Sora. Of course, Yozora is skeptical. Why would he believe a stranger? MoM tells Yozora not to take his word for it. That fate would soon bring them to meet each other and that's when Yozora would know.
And soon enough, by fate, Yozora does meet Sora, just as the stranger said he would. This is proof enough to him that Sora is the person he must be looking for.
But to Yozora, Sora still /feels/ like a stranger. Which is why he has no hesitation in taking Sora down...in order to save him. But then why does he treat Sora like someone he cares deeply about afterwards? I think I have...a pretty good hunch about that.
See, Yozora may have forgotten nameless star, but he might not have forgotten every memory involving her. In his remaining memories there's a blank space where a person should be. And there's one memory that is more important than any other; an oath.
Remember how in the Verum Rex trailer, Yozora is doing everything he can to try to save the girl- who, for some reason, seems to have a special- and destructive- power she can't control. I believe he made this oath to her, that he would save her from this fate.
But... if Sora's memories of Riku are becoming intertwined with Yozora's...what if there was a memory from Sora similar enough to start blending with this most important memory? Oh yeah, you know where this is going.
Full-on Chain of Memories necklace theory.
In CoM, Namine used a real memory between Sora and Riku, mixed and melded their perspectives on the memory and inserted herself in each of their places. Sora and Repliku both believed they had made a promise to protect her.
Another important element of the memory is the good luck charm (which, in the novel, is worn as a necklace!). In the true memory, this good luck charm is heavily hinted to have been Sora's necklace, given to him by Riku. But under Namine's influence it takes the form of a star.
How interesting...a necklace with a star pendant, given as a reminder of an important promise, maybe even an oath... Ah. Someone we know wears a star necklace.
I'm fairly confident that this memory is getting blended with Sora's, making Yozora believe he made an oath to save Sora: In Eng it's clear enough, but in JP it's undeniable. Yozora says the same thing to Sora as in the kingstagram post tagged #/oath (thanks @databoyreekoo!)
Not only that, there's something else interesting about that kingstagram post. Where there is usually a portrait and name, there are question marks. We've seen this before; when more than one person has said the quote in the post. This means these words aren't just Yozora's.
In my mind, there's not many other plausible explanations for Yozora's strange behavior based on the little information we have available and previous concepts. Treating Sora as a stranger in one moment, and as someone important to him in the next... Yozora is at a point where he doesn't "remember" Sora (in a false memory) but his memories have been messed with enough to the point where he believes Sora must have been the person missing from his memories, the one he made an oath to save.
tldr; Yozora is in the position Sora was in during CoM-searching for someone important to him and slowly losing his memories which are getting replaced with memories that don't belong to him, and Shibuya is his Castle Oblivion.
Adding these tweets as evidence:
*remembers that red pulsing light on Yozoraās face in the secret ending, like a heartbeat* oh
I...I can't believe I never connected this before. At the end of the secret episode we see Yozora wake up, (probably) meaning what we saw beforehand was /his/ dream. And he is able to summon Sora's heart station. Because who is inside his heart? Sora. The reason there are things like the crown symbol and an ad for Destiny Islands in the secret ending version of Shibuya, the dream Shibuya? Because it contains Sora's memories. The reason Yozora can't tell whether his inner voice in that moment was his own or Sora's.
ALSO in khmom Xehanort tells Kairi that the world they are in (that version of the Final World) is constructed from her memories/heart as she sleeps. It stands to reason that the nighttime final world is constructed from Yozora's heart as he sleeps.
Love how- if Iām reading this correctly- they basically tell you whose heart it is that has replaced Yozoraās according to the nameless star. Sora literally just shows up in a world as Yozora. Everyone recognizes him as Yozora, even the Verum Rex game he gets trapped in.
Sora necklace has suddenly changed to look a lot more similar to Nameless Star's necklace :)