Nobodies are born when a Heart is swallowed by darkness and becomes a Heartless. They are the remaining parts left behind by the heart: the body, giving a Nobody form, and the soul, giving the Nobody life. However, only those with a strong will such as Sora are actually able to continue on as Nobodies. Those with the strongest wills manage to retain human forms (under special circumstances, sometimes the Nobody is formed with some slight changes to their appearance), while the rest possess malformed, barely humanoid appearances, such examples as Nobodies like the Creeper and Dusk. Therefore, the more humanoid the Nobody, the stronger its will. The primary stated requirement to wield the keyblade is an exceptionally strong heart. The members of Organization XIII were all defined by their exceptional willpower and the strength of their lost hearts. Xehanort also seemed to focus his attention on a keyblade wielder as his first host, considering first Ven and then turned his focus to Terra. Given that his purpose for Organization XIII was to gather a dozen extra hosts, it makes sense that he would focus on individuals with the makings of a wielder. He originally intended to use Riku, but turned his attention to Sora/Roxas in the end. Lea has gained the ability to wield a keyblade, most likely because of Ven, Roxas, or Xion. It stands to reason if three (technically four, if you count Xemnas) members can wield, perhaps others from that group have the same makings.