Replica Program

The Replica Program was a planned series of experiments conducted by Vexen under Xemnas's orders, with the apparent help of Naminé. He attempted to make Replicas of Keyblade wielders Sora and Riku in the event that the original Sora proved to be useless to his plans for Kingdom Hearts. The first replica created by the program was Xion (an anagram for "No i"). However, after Xion's defection and her death by Roxas, Xemnas discontinued the program. In the Secret Reports it mentions another inferior numberless replica that was taken to Castle Oblivion. The Secret Reports in Kingdom Hearts 358/2 Days also reveal the existence of other unused replicas.

Xion
Xion was created by Vexen out of Sora's memories. She held these memories which would stop Sora from being able to awaken from his slumber. Xion, being made out of Sora's memories, could wield a Keyblade, something the Organization wanted. This would make it so the Organization had two among their ranks that could wield the Keyblade and collect hearts to form Kingdom Hearts. She was seen by some as only a puppet, while only Roxas and Axel called her a friend. She was destroyed by Roxas, and the program was stopped shortly after her death.

Riku Replica
The Riku Replica was created shortly after Xion was created by Vexen. He created this replica after sampling the real Riku's memories. Vexen used this replica to fight both Sora and Riku. The Riku Replica always knew he was the fake one, but always fought to get rid of Riku, what the Organization wanted. He met his demise by Riku, and was swallowed by the darkness after being defeated.