Replica

A Replica (レプリカ), (also called clones, fakes, or toys by Larxene) is an artificial copy of another being. The first Replica seen in the series is Riku Replica in Kingdom Hearts: Chain of Memories. A second Replica is later revealed to be the fourteenth member of Organization XIII, Xion. Also, in the manga adaptation of Chain of Memories, Vexen makes a vast amount of Replicas of himself, which survive into Kingdom Hearts II.

Nature
Replicas are synthetic copies of other people. The exact method of creating a Replica is unknown as of yet. Also, Replicas have been shown to be able to be created out of different materials. Riku Replica was created using data collected by Vexen from his fight with the real Riku. He believed he was the real Riku due to Naminé's memory manipulation. While Xion, an imperfect Replica of Sora, was made from some of Sora's memories that "leaked" while he was in a Memory Pod. Xion was so different from Sora because the memories she was made from took the physical appearance of Sora's most important memory; Kairi. Replicas, like Nobodies, fade into Darkness after they are defeated.

Replicas have emotions, and therefore have the ability to feel. Most Replicas have a mysterious, stoic nature and very seldom give straight answers. Also, a trait shared by all Replicas show in any version of the series is that they are very sensitive about the fact that they are copies. Riku Replica, upon learning he was a Replica, constantly referred to himself as "fake" and became obsessed with trying to become a different being than Riku. In the manga, the Vexen Replicas are very quiet and sad compared to the original Vexen, telling the Riku Replica that "we are all failed experiments." Xion goes temporarily insane when she finds out she is a Replica. She becomes so deranged she decided to try to kill Roxas.

Abilities
Replicas seem to have a few unique abilities. Replicas appear to be able to fight as soon as they are created, not needing to learn fighting techniques or become stronger. This is most likely a trait passed down from the person they were created from. The most prominent ability Replicas have is the ability to absorb the life-force of other beings, increasing the Replicas' overall power and allowing them to use any unique abilities the being absorbed may have had. In fact, the main reason for Xion's creation was for her to absorb Roxas to become Sora. Also, Riku Replica absorbed Zexion in Kingdom Hearts Re:Chain of Memories, giving him the ability to use Zexion's power.

Trivia

 * Because Xion would have become Sora if she had successfully absorbed Roxas, it is unknown why Riku Replica did not transform into a different entity after he absorbed Zexion.
 * Every Replica ever created was made by a member of Organization XIII.