You have magic too? [Oh god. That was one less thing he was going to have to explain then since so many people seemed to be unfamiliar with magic as a real thing.]

Heartless...they're creatures that manifest when someone loses their heart. Their heart becomes a Heartless--well, the darkness inside it twists the heart to form them. [It's confusing, he knows.] They got their name because they don't feel anything and just attack everything. Try to devour worlds and steal more hearts to create more Heartless. It's..a never ending cycle.