he was a copy of the original riku to carry out the orders of the organization. i think he was eventually destroyed by the original riku. he is definently not in kh2.
kh.co.uk:
Riku Replica
A replica of Riku created by Vexen from the real Riku's data.
Unlike the real Riku, he wields darkness freely, but is burdened by the fact he is an imitation.
Through Naminé's power he was infused with false memories and made to believe he was the real Riku.
The truth has wounded him deeply.
also
Riku Replica
He was created by Vexen using data from his battle with Riku. He is first sent to dispose of Riku, only to retreat. Afterward, his memories are manipulated by Naminé, beleving that he is the real Riku and that Sora will hurt Naminé. After Sora's final battle with 'Riku', Larxene reveals the truth, which Riku Replica overheard. He returned later to save Naminé from Marluxia and vanishes at the end of Sora's story. He is picked up by Axel (who appears after he fakes his death) and is sent to kill Zexion so that he could become "real". Riku Replica understood, however, that the only way he could become real was by killing the original. He was eventually defeated by the real Riku, and died with his last thoughts being where someone like him (having a fake heart) would end up.
He is also called Nisemono in the Japanese version, which means imposter.