Xion theories.

[Lifes.Lover]

Even if it is a promise he made, she practically forced it out of him. So, in the end, it doesn't matter- the Oathkeeper is a symbol of the memory he has of the promise and of her.

 

[#1 Xemnas]

wait, i thought the oath keeper was just the symbol of the promise. where did memory come from?

 

[Lifes.Lover]

It is most often thought that keychains, which are what make the keyblade look all splediferous and stuff, are made up of the memories of a person or place that comes into contact with the keyblade wielder- i.e. Sora.

 

[#1 Xemnas]

isn't that only theory? i always thought, though it was never said, no wait i think someone said "take this" or "to remember me by",it was an actual keychain given by characters. i.e. jack gave sora the pumpkin king, or how sora basically turned kairi's lucky charm into a keychain.

 

[Lifes.Lover]

They give them something, and that something turns into a keychain that they can use. Hence why Kairi's thallassa shell charm given to Sora by her, with the promise attached, was turned into a keychain.

 

[Smile]

People giving Sora things to be turned into Keychains = Gameplay mechanics until further notice. It added effect, but c'mon, people couldn't make a Keyblade out of nothing, why should they Keychains unless they're Moogles and you had to be their slave for an entire game?
It's Sora's Memories, no doubt, and while I know that's shooting myself in the leg, it also ties in with how come you only really get one Keyblade in CoM - Sora lost the Memories he used to make the rest, and then the Memories were jumbled up so he had to use the cards instead of the actual blade.