That was my... my dark side. I gave in to the darkness once. And ever since, it's chased me around in one form or another. The Seeker of Darkness who stole my body... a puppet replica of the shadows in my heart... and now, I'm facing me.Riku to Jiminy Cricket
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.
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 the 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.
Like a majority of the Thirteen Seekers of Darkness, Dark Riku primarily appears wearing a black coat. During the Second Keyblade War, Dark Riku uses Dark Mode and the bodysuit given to him.
Having had his heart collected in the past by Young Xehanort, the Riku Replica is brought to the present to serve as one of the Thirteen Seekers of Dakness. Given a new replica body to inhabit, the newly dubbed Dark Riku was tasked along with Ansem, Seeker of Darkness to enter the Realm of Sleep to antagonize the real Riku by remiding him of his past misdeeds and weakness in having surrendered to the darkness.Ended up in Prankster's Paradise, Dark Riku finds himself in Monstro. To envoke deja vu, Dark Riku lures Pinocchio away 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 golden 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". Dark Riku then 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 soon walks hooded in his black coat with the rest of the Thirteen Seekers of Darkness as they confront the Seven Guardians of Light at the crossroads of the Keyblade Graveyard. He is seen again in the Skein of Severance, where he battles against Riku with Ansem and Xigbar in the Trail of Valediction. As Ansem takes his leave from the battle shortly after Sora's arrival, Riku attempts to pursue him, although Dark Riku kicks him back midair. Once Riku and Sora recover, the battle resumes.
After Sora and Riku defeat Dark Riku, it is revealed to Riku that Dark Riku is not his past self possessed by Ansem, but rather the time-displaced Riku Replica, his 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 his replica body, wanting it to be used for Naminé instead. Afterwards, both replicas vanish in a spire of both light and darkness as Dark Riku's heart returns to its proper place in time.
Like Riku when he gave into darkness and his original self as the Riku Replica, Dark Riku wields the power of darkness, and enters a form known as Dark Mode to channel his dark powers. In combat, most of his abilities are identical to Ansem's, albeit color-swapped into a purple color as opposed to Ansem's dark red.
In conjunction with his powers, Dark Riku wields the Soul Eater, which powers up with the use of darkness.