[Roxas makes a grunt of acknowledgement, nodding. Oh, that was true. Even if all he was doing was observing a new world, there was always Heartless to fight.]

Heartless are creatures of darkness that prey on the light in people's hearts. They're kind of like the Erasers here, in a way, only instead of attacking stories, they target the hearts of people. Of worlds. I'm kind of surprised they haven't made it to this world, honestly; they're pretty persistent.