Dark Riku is the heart of the brainwashed Riku Replica from the past. He was given a new replica body and was later infused with a fragment of Master Xehanort's heart to serve as one of the Thirteen Seekers of Darkness.
Kingdom Hearts III
A member of the real Organization XIII.
He looks exactly like Riku, except a year younger, and he wears the Organization's black coat.
Dark Riku makes a small appearance during Riku's visit to Prankster's Paradise, having taken Pinocchio from Geppetto. Riku and Jiminy Cricket later encounters the hooded Dark Riku with Pinocchio in the depths of Monstro. After Dark Riku allows Pinocchio to reunite with Jiminy, Riku questions him on his identity. Dark Riku soon pulls down his hood to reveal his face to Riku, much to the latter's surprise. Before Riku could question him further, Dark Riku leaves through a corridor of darkness.
Later, a hooded Dark Riku appears alongside the other Thirteen Seekers of Darkness at Where Nothing Gathers before he and the others are forced to temporarily withdraw due to their allotted time running out.
Dark Riku is later revealed to be a member of the Thirteen Seekers of Darkness, having obtained yellow eyes to represent his housing of a fragment of Master Xehanort's heart. He appears before Sora and Big Hero 6 in San Fransokyo, using the Blocks to gather data on negative emotions. He gains possession of the first Baymax's fighting chip and uses the data, along with the retrieved body of Baymax, to create Dark Baymax.
Dark Riku reappears in the Keyblade Graveyard, coming face-to-face with Riku for a second time. Riku, believing that he was facing his own past self, counters the Replica's claim that his possession by Ansem was his "greatest hour". He summons a gigantic dark effigy of Master Xehanort, and Xigbar appears and tells him to unleash the Heartless within the effigy. Dark Riku disappears shortly after.
Dark Riku is seen again in the Skein of Severance, where he battles Riku and Sora alognside Ansem and Xigbar in the Trail of Valediction. As Ansem takes his leave from the battle, and as Riku pursues, Dark Riku knocks him back. After Sora and Riku defeat Dark Riku, it is revealed to Riku that Dark Riku is the time-displaced Riku Replica, its heart residing within one of the Replicas created by Vexen. The present version of the Riku Replica emerges from within Riku, and forces Dark Riku out of its replica body, wanting it to be used for Naminé instead. Afterwards, both replicas disappear.
As he is from the time in which he was brainwashed in Kingdom Hearts: Chain of Memories, Dark Riku considers himself to be the "real" Riku, and 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 all those he meets to believe that he is Riku during the time he was possessed by Ansem, pridefully referring to that time as his finest hour when talking with the real Riku.
Dark Riku is physically identical to a younger Riku due to being Riku Replica from his time in Castle Oblivion; sporting the same skin tone, and shoulder-length silver hair. A key difference between the two is the eye color, with Dark Riku's eyes being golden - a sign of him being a vessel for a piece of Xehanort's heart, though Dark Riku lacked this feature in his initial appearance.