Riku-Ansem

Riku takes on this form after he failed to capture Roxas under DiZ' request to awaken Sora. He summoned the dark power of Xehanort's Heartless in order to use it against Organization XIII. His first mission after this transformation is to capture Roxas, which he succeeds.

He continues to remain in this form throughout Kingdom Hearts II, and most people believed that he was Xehanort's Heartless until Kairi found out that he was Riku and revealed his identity to everyone.

After Sora and company catch up with Ansem the Wise, he uses the Kingdom Hearts Encoder to attempt to turn Kingdom Hearts into data. This plan backfires, and during the explosion Riku is transformed back into his normal form.

However, it also reduced his power of darkness, as revealed when they attempted to leave the World That Never Was and Riku failed to open a Corridor of Darkness.

It is not know if he ever uses this form again.