>>527409234
they're basically just plague animals that breed out of control by infecting hosts. People with strong darkness are able to order them around and they listen to whoever has the most darkness in the room (or maybe maleficent & the org both created their own armies with darkness that they control?).

as for them matching the worlds, I thought the explanation was that ansem designed the emblem heartless but they probably take forms to match the world on their own like donald's magic. he might have just stamped them with emblems to differentiate the ones that will listen to commands.

Also worth noting that heartless are especially drawn to the keyblade wielder as its the only weapon that can defeat them (except it isn't really)