A heart... it's a thing of great power. Sometimes, though, people lose their hearts. They may be consumed by darkness, or given up somehow... A heart that's consumed by darkness usually becomes a creature called a Heartless. That's actually what I thought happened to Disciple, and why I was so worried; Heartless can normally only be beaten for good with a Keyblade. Anyways, when a strong-willed person loses their heart, the empty shell left behind begins to act on it's own.

That creature's called a Nobody. That's what I am; someone without a heart, born when someone lost theirs.

[He hopes this all makes sense. He's never had to really explain this to anyone before.]

Sora's the one who I came from. And he's the reason I have a Keyblade, where I'm from. If he hadn't lost his heart... I wouldn't exist.

[Technically, Nobodies weren't meant to exist, but... well.]