Dark Riku

Dark Riku is Riku Replica's time-traveling heart in a new, replica body, and was originally believed to be the real Riku from back when he was possessed by Ansem, Seeker of Darkness. He is a member of the real Organization XIII, infused with a fragment of Xehanort's heart. After Dark Riku is defeated in Keyblade Graveyard, his body is used to resurrect Naminé.

Kingdom Hearts 3D: Dream Drop Distance
Shortly after Vexen was recruited to perfect the replicas to serve as vessels for hearts from the past,  the heart of the Riku Replica was brought from the past and placed in a new replica; the new "Dark Riku" took Demyx's place in the Real Organization XIII.

Dark Riku was sent to the Sleeping World of Prankster's Paradise to unnerve the real Riku. He leads Pinocchio away, luring Riku to him; letting the boy go, Dark Riku removes his hood and leaves without a word.

After being infused with part of Xehanort's heart, Dark Riku joins the rest of the Organization in Where Nothing Gathers. Once Sora is saved from becoming the thirteenth vessel, Dark Riku retreats along with the rest of the Organization.

Kingdom Hearts III
In San Fransokyo, Dark Riku conducts an experiment to see if a heart can be recreated from data. He uses a chip designed by Hiro Hamada to forge a heart born from conflict, and with it, he is able to unleash Dark Baymax upon Sora and company.

At the Keyblade Graveyard, Dark Riku confronts the seven guardians of light and demonstrates his superior strength for them after mocking the real Riku. Dark Riku is ultimately defeated after a clash with Sora; his heart is extracted by another version of himself, who had come to reside in the real Riku's heart, leaving behind an empty replica vessel.

After Master Xehanort's defeat, the empty vessel is used to restore Naminé to life.

Design
Dark Riku is a replica of the real Riku's younger self, and is thus physically identical to him. His eyes are initially aquamarine, later turning yellow after Xehanort places his heart inside him. Dark Riku primarily wears a black coat, entering Dark Mode to challenge the seven guardians of light.

Personality
As in Kingdom Hearts: Chain of Memories, this version of the Riku Replica considers himself to be the "real" Riku, superior to the actual one in every way. He acts just as his counterpart did when possessed by Ansem: arrogant, cruel, and remorseless. For much of Kingdom Hearts III, Dark Riku allows Sora and his friends to believe he is the real Riku's past, corrupted self.

Abilities
Like his other self, Dark Riku has all the real Riku's powers. He fully embraces the darkness in battle, and he wields a version of the Soul Eater.