Riku Replica
The Riku Replica's first and only appearance was in Kingdom Hearts: Chain of Memories. He was created by Vexen of Organization XIII. He was killed by the real Riku in that same game.
Beginning
The Riku Replica created by Vexen to test the real Riku's power, and if possible, coax Riku into turning to the darkness. However, the Riku Replica this early on in his life (though he looked just real Riku and had the power of darkness on his side) to be capable of defeating the real Riku. After this battle, however, Vexen took the replica above ground.
Sora battles
When Vexen took the Riku Replica above ground to face Sora and (in the Replica's plans) to train him to be able to defeat the real Riku. However, at the suggestion of Larxene, the Replica's memories were rewritten to resemble those of the real Riku, though the replica objected to this idea. The replica fought Sora four times before Naminé finally stopped him from killing Sora after the end of the fourth battle by breaking his heart. At this moment Larxene revealed the truth. The Replica was out for a little while and did not reappear until Marluxia was threating Naminé and Sora. He aided Sora during the battle and then agreed to watch Naminé while Sora, Donald, and Goofy fought Marluxia. After Sora told him that it didn't matter if he was a fake Riku because he had his own heart. The replica seems to have been rather touched by Sora's words. However, meeting up with Axel after he left Sora, he still had one thing left to do: Defeat the real Riku.
CoM manga
Riku's replica never fights the real Riku and is not killed. However, he encounters Riku, and tries to make himself look taller. When Riku blatently ignores him, the Replica visits Vexen's grave, where he encounters Vexen clones, who keep following him. He also takes a bit off piece of a rice cake, and puts a chain on it, pretending that it's a "new good luck charm." Referring to Naminé giving him the fake luck charm.
Riku Replica VS Riku
Under Axel's notion that he (the replica) needed more power to destroy the real Riku, the Replica killed Zexion. However, Riku's power had grown as well. He now used the power of darkness with no resentment. The two fought ferociously but in the end the real Riku prevailed. In the end the replica aknowledged what he had known from the start: He was but a mere copy.
With his dying breath, he asks Riku where his Heart would go to after his death. Riku comforts him with the notion that his (the Replica's) heart would go to the same place his would. The Replica smirks and replies "How original" before fading away.
In Combat
The Riku Replica fights with the same style as the real Riku, but his sleights take less power than Riku's. For example, Riku can only use Dark Aura with three 9's, a total of 27. However, the Replica can use any three cards, say 4, 7, and 8 to perform Dark Aura. He is also more aggressive, which leads to some open moments during his battles.
Card
See: Riku Replica (Enemy Card)
Personality
The Replica is more aggressive and protective than Riku, which is exactly what Xehanort's Heartless was looking for. He is also more stubborn. At the end of his life, the Replica became much more like the original, he became sullen and mellow. He was quick to anger, and often lashed out at anyone who talked back to him, even if they overpowered him. He seemed to be as powerful as Riku was at Hollow Bastion, when under Ansem's Possession.
