Main monster enemies in Kingdom Hearts II, resulting from when a strong-willed person loses their heart and is split between their Heartless (Kingdom Hearts) and the remnants of their body and soul. When that person's Heartless and Nobody are defeated, they return to their original selves.
Organization XIII is a group of thirteen powerful Nobodies who have control over a specific class of lesser Nobodies, generally based upon their weapon and/or specific element. For example, Axel can use Assassins, while Xigbar controls Snipers.