A heartless is a creature born from the darkness. They go after people's hearts, and if they get you, your heart can end up turning into one. Nobodies are what's left over; the mind and the body, without a heart. Though I guess Nobodies can regrow hearts over time, if they make connections with people.

[He shrugs.]

Sora's got this way of bringing people together; he's made connections with lots of people in lots of worlds. It's not special like magic special, he's just kind of a cool guy.