That voice

It's Kingdom Hearts itself!!!
. . . .
Actually I wouldn't have a clue. There is no sound to it, just text. But it is talking to Keyblade wielders so perhaps it could be a or THE original Keyblade Master/s.
Could be Terra for all I know, and what do I know? Very little.

 

I believe it is the heart of the worlds. They speak as one to inform the key blade warriors of what is expected of them. But I'm just speculating. Still, this theory makes the most sense.

 

you know, that's a pretty good theory!

 

Huh? Which part? Give me the link.

 

Oh, that one XD

Well... I assume that it must be the Keyblade itself.

But in KH2, I think it should be Riku since before you fight Twilight Thorn, there is Cloaked person in there.

 

I think the cloaked guy was Roxas himself, his darker side. It would make sense that he would fight a part of himself, his memory.

 

Makes sense since when Sora's Deep Dive/ Dive Heart is his shadow/dark side

 

So the Heartless is Sora's Shadow
Twilight thorn is a part of Roxas's memory or form as a true nobody
The voice may as welll be the keyblade, but for roxas maybe it's the same, but it talked differently since roxas is sora's other side? i"m not sure about that one lol

 

The Heartless is called Darkside. They shown lots of themselves in the ending of Kingdom Hearts, so its unknown if that's really Sora's shadow

And the Twilight Thorn is basically the Nobody of Darkside. I'm pretty sure about it

 

Ah, that makes sense, but if that dark side had a nobody then maybe all those other Dark sides are from certain people or something like that.