Summary
Kingdom Heartsis known for its layered approach to storytelling; parallels and symbolism are everywhere if you’re willing to look for them. Sora’s crown necklace, a consistent piece of his design that he’s rarely seen without, could also be much more important than it initially seems.
Crowns are seen almost everywhere in the series (it is calledKingdomHearts after all), and the symbol is seen onthe Oblivion Keyblade, which represents Sora’s memories of Riku. The crown keychain on the Keyblade looks very similar to the necklace itself, as both crown pendants are connected by chain links.

The symbol is also seen on the Kingdom Key, and the outline of the Keyblade’s teeth is shaped exactly like the crown pendant. While Sora wields it, it was originally Riku’s Keyblade, passed down to him by Terra, and then passed to Sorawhen he touched the light of Riku’s hearton the night that Destiny Islands fell to darkness. The crown symbol is once again associated with Riku.
These details have inspired what’s known as The Necklace Theory, postulated byTumblr user blowingoffsteam2.

What Is The Necklace Theory?
The Necklace Theory suggests that Riku gave Sora the crown necklace when they were children, similar to how Kairi gave Sora a good luck charm in the first game.
The connection between the Oblivion Keyblade and the crown necklace is already a big visual parallel and enough to give some credence to the theory, buta comment in the Kingdom Hearts Ultimania about the Oblivion Keybladesheds light on this even further:

Its development name was the ‘Riku Keyblade.’ In early stages we planned to have Riku give you a keychain, just like the lucky charm, which is why the keychain is a different colored version of Sora’s necklace.
While Riku explicitly giving Sora a keychain was ultimately cut from the finished game, it’s possible that this event still happened; we just didn’t see it.

The Meteor Shower Memory
Oblivion’s Japanese name is ‘Sugisarishi Omoide’ , which translates to ‘passing memories’ or ‘memories of the past’, and even its English name is associated with forgetting, as we see with Castle Oblivion.
From this, we can infer that Sora may have forgotten a memory related to or including Riku, since it’s Riku who is tied to this Keyblade.

The Necklace Theory suggests that this is a memory that plays an important role in Chain of Memories — a promise to protect Namine on the night of a meteor shower on Destiny Islands.
We know Namine can’t be the real person in the memory, since she is actively implanting herself into Sora’s memories and taking the place of others to convince Sora that she is his most cherished person and who he needs to save in Castle Oblivion.
Sora first remembers this promise in Twilight Town and recalls it toDonald and Goofy. In the very next scene, Vexen taunts Sora by telling him that he has been to Twilight Town before, despite having no memory of it himself (because Roxas has been there).
It is on the other side of your heart that the memory of this place exists. It is your heart that remembers.
Placing these scenes directly after one another is certainly telling. Sora’s memories of the meteor shower promise resurface at the same time as his ‘memories’ of Twilight Town, showing just how deep this memory was hidden — so deep that they’re on the ‘other’ side of his heart, as Vexen would say.
Later in the game, Sora meets a replica of Riku created by Organization 13 and implanted with Riku’s memories. Both Sora and the Riku Replica believe they were the one to promise to protect Namine on that night, which results in a heated argument between them:
“Sora, I made a promise to Namine. I promised to… keep her safe.”
“YOU did?”
“There was a meteor shower… this one night when she and I were little. Namine got scared and said, ‘What if a shooting star hits the islands?’ So I told her: ‘If a shooting star comes this way, I will protect you.’”
“You made a promise! With a toy sword!”
“What?! How do you know about that?”
“Because… that was the promise I made to her that night! I would protect her! I said it!
“Don’t lie! You weren’t the one there that night!”
Sora has no idea that this isn’t the real Riku at this point in the game.
Most people, with good reason, assume the real memory took place between Sora and Kairi. But is it really that unlikely that it was a shared memory, manipulated to push the Riku Replica further from Sora, and Sora from who he believes is the real Riku?
Namine herself explains how she uses her powers to manipulate Sora’s and the Riku Replica’s (i.e. Riku’s) memories: “I don’t actually erase any memories, just take apart the links and rearrange them.”
She also gives a clearer explanation of her powers in the 358/2 Days novel, “But — all I can do is rearrange fragments of memories. I can’t rearrange things that aren’t already there.”
Going by these explanations, it’s not unreasonable to assume that the real Riku also had this memory, which was planted into the Riku Replica, and was then manipulated by Namine to turn him further against Sora. It’s pretty clear that Namine can’t create memories out of thin air, so she rearrangedsomethingin Riku’s memory to make him believe he made this promise.
Organisation 13 also refer to Sora’s memories being “shuffled” — like a pack of cards, exactly like Chain of Memories' mechanic. Namine shuffles what’s already there — she doesn’t add to it.
The memory also took place on the night of a meteor shower. The only meteor shower on Destiny Islands (that we know of) happened when Kairiarrivedon the island. It’s impossible for her to arrive on the island and be in the memory at the exact same time.
The Forgotten Memory Within A Memory
While Sora does remember the event of the meteor shower as he goes through Castle Oblivion, it seems that he may have forgotten an important part of it: Riku giving him the crown necklace and promising to protecthim(rather than Sora promising to be the protector), as represented through the matching crown pendant on Oblivion.
Axel hints towards Sora having forgotten a memory deep within his heart after your first fight with him in Chain of Memories.
Our most precious memories lie so deep within our hearts that they’re out of reach. […] You have lost sight of the light within the darkness. And it seems that you’ve forgotten that you’ve forgot. […] Would you like me to give you a hint?
Axel actually gives Sora (and the player) the hint within his body language — he extends his hand towards Sora, in the exact same way that Riku does, as seen in many of the games’ openings. It’s possible that he’s extending his hand as if to offer something — like a necklace, perhaps?
Interestingly, Riku’s poem from Chain of Memories also highlights a “forgotten promise.”
Beyond the path without you
We may have walked side by side,
And though our paths may not cross,
When I arrive at where you are,
we may not appear to be as we were…
But we’ll make another promise to keep.
Sora may have forgotten Riku’s promise to protect him, but Riku hasn’t. This is reinforced throughout Riku’s entire series-spanning character arc — he has always strived to achieve “the strength to protect what matters” and been determined to protect Sora in any way that he can.
Chikai, the Japanese counterpart to Don’t Think Twice for KH3, includes the lyrics,“No more promises / Those are for making someone happy / What I’m saying now isn’t some second-hand opinion / It’s not even a promise, it’s an oath”. Having grown older, perhaps Riku now wants his devotion to be something stronger than a childhood promise.
Though it’s not confirmed whose perspective the song was written from, it’s an interesting connection.Nomura said ‘Sanctuary’ was written about Sora and Riku, after all, so the possibility is there.
The Fake Charm
As part of the Organization’s plan to manipulate Sora, Namine transforms the good luck charm he received from Kairi in the first game into a charm shaped like a paopu fruit and makes Sora believe that Namine gave it to him on the night of the meteor shower.
Interestingly, when talking about this charm, Sora says, “Namine said she had it ever since she was a baby.” She had it ever since when now? Kairi is shown making the good luck charm in KH1, just before she, Sora, and Riku plan to head out on the raft.
We already know that Namine can’t create memories from scratch, and can only switch them around. So, who does this charm originally belong to, who also had it since they were a baby?
Would you like me to give you a hint?
In the Chain of Memories novel, the memory is brought up again, and there’s some extra detail about what the charm given on this night was.
“If any falling stars come this way, I’ll protect you!” He grinned at her.
“…Promise?” Namine said in her tiny voice.
“I promise,” he told her steadily.
Then, at last, she smiled… and held out a star-shaped pendant for him. “Here… So you don’t forget our promise.”
It was a charm made out of a paopu fruit.
“They say that if you wear this kind of fruit, you’ll never ever be parted from the one you love,” she said.
“That’s just…,” the boy started but took the pendant and looped it around his neck.
“So, no matter what happens, we’ll always be able to find each other again.”
The charm was “looped […] around his neck.” Sure sounds like a necklace, huh?
What Could Sora’s Forgotten Memory Mean For KH4?
Going into KH4, Sora has yet to recall this memory hidden deep within his heart. He’s in Quadratum, separated from everyone he’s ever met, so could it be time for Sora to do some introspection, and uncover those precious memories that he’s forgotten like Axel said?
There’s even somewhat of a parallel here of Riku heading to Quadratum to look for Sora, turning the events of Chain of Memories — Sora looking for Riku in Castle Oblivion — on its head.
Only time will tell how things play out in Quadratum, but it’s possible that Sora’s memories may become important once again.