Heartless move like animals and creatures that could feasibly exist and Nobodies move like a poorly-done sfm animation with shiny, rubbery textures no living humanoid should have. End of the World is all crags and sparkling waters and strange plants and life and everything from everywhere mashed together and The World That Never Was is all hard edges and neon lights and the illusion of a city where not a single person lives and it is pristine and perfect. A heart is connection and growth and a hollow body is stagnation. As a final boss Ansem turns into a massive inorganic structure (boat) made of organic material (flesh) and Xemnas turns into a massive organic structure (dragon) made of inorganic material (buildings). The former, the heart and Heartless and End of the World, is about the raw substance, and the latter, the body and Nobody and The World That Never Was, is about the appearance. and you need both
