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
Riku Replica's heart is brought from the past and placed in a new vessel to serve as a member of the Real Organization XIII as "Dark Riku".
Prior to receiving a fragment of Master Xehanort's heart, Dark Riku is sent to Prankster's Paradise to unnerve Riku by recreating his kidnapping of Pinocchio. When caught, Dark Riku reveals himself and silently leaves via a corridor of darkness; his appearance prompts Riku to recall the original Riku Replica.
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 after the Organization's attempt to induct Sora is stymied, 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 the current version of himself who had been coexisting within Riku's heart, Riku Replica, leaving behind an empty replica. Dark Riku's heart returns to his past while his present day self fades away; the vessel is then collected by Demyx, who takes it to Radiant Garden.
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.
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 possessed self from the past.
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. With the fragment of Xehanort's heart within him, Dark Riku's powers are stronger than his other self; able to manifest darkness in the shape of Xehanort and summon Heartless and Nobodies from it.
Dark Riku is also capable of using Dark Corridors to reach alternate dimensions outside the three main realms, being able to retrieve the original Baymax. Shown to be good with mixing magic with technology, Dark Riku attempts to craft an artificial heart using Baymax's old combat chip by imbuing it with negative emotions from the victims the Heartless attack and the positive emotions from having the Darkubes fight Sora.
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