DoorToDarkness

So for the unintiated the character in this meme is Namine. A character who plays a very key role in the second ever KH game, Chain of Memories.

Without getting too deep into her origins, Namine has the ability to alter peoples Memories. This is a major plot point in CoM where, as a prisoner of Organization XIII, She's forced to alter Sora's memories as part of Marluxia's scheme to make Sora his puppet to overthrow the Org leaders. She's also made to put an altered copy of Riku's memories into Repliku, a clone of Riku.

In both cases, the main thing she alters, aside from inserting herself generally into their memories as an old friend, is a very specific memory.

In the Memory the kiddos are watching a meteor Shower on Destiny island. Lil Namine is scared a shooting star will hit the island, and then the boys, either Sora or Riku/Repliku in their respective heads, promise to always protect her, and Namine gives them a paopu fruit shaped charm as a symbol of the promise.

In the story of Chain of Memories, this altered memory is used to keep both Sora and Repliku obsessed with saving Namine so they play into Marluxia's plans. And to the average player youd probably just cone away thinking it was a false memory Namine made up.

Except when all is said and done Namine explains how her powers work and that She CANT create New Memories Wholesale. Only alter and rearrange existing ones.

So, the theory begins, because this means that both Sora and Riku have a Memory of that Meteor shower, where somebody was scared, someone promised to protect the scared person, and someone was given some sort of good luck charm as a symbol of that promise. Its a memory they both have. And it cant involve Kairi because, fun fact, the meteor shower they were watching happened the night Kairi arrived on Destiny Island, before they met her.

See where this is going?

So the theory goes that in the original unaltered memory, Sora was the one who was scared of Shooting stars hitting the island. Riku was the one who promised to protect Sora no matter what, and then He gave Sora a "Lucky charm" to symbolize that promise. The Charm? SORAS CROWN NECKLACE.

Thats the basics.

Theres a lot of supporting evidence for the theory that people with far more time than me have complied elsewhere. The bit i remember off the top of my head is that Oblivion, the keyblade symbolizing Sora's memories of Riku, has a black crown that matches his necklace as its keychain (paralelling the wayfinder charm on Oathkeeper) and its original name in japanese is something like "forgotten memories"

Like i said theres a lot more evidemce out there complied by people much more eloquent than I.