Dark Riku

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

Kingdom Hearts 3D: Dream Drop Distance
Dark Riku reveals himself to the real Riku inside Monstro. He quickly vanishes inside a corridor of darkness, but his appearance prompts Riku to recall the "imitation that was copied" from the darkness in his heart.

Later, Dark Riku joins the other members of the real Organization XIII in Where Nothing Gathers at The World That Never Was. His time soon runs out, however, and he is returned to his proper place in history.

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.

After Master Xehanort's defeat, Dark Riku's body is used to restore Naminé to life.

Design
Dark Riku is a replica of the real Riku's younger self, thus he is physically identical to him. While his eyes were initially aquamarine in coloration, they turn yellow after Xehanort places his heart inside him. Dark Riku primarily wears a black coat, but he later enters 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.