Dark Riku

"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, only thinking highly of himself and his control over darkness; his interactions with Big Hero 6 also displays him being incredibly sarcastic and egotistical. For all of Kingdom Hearts III up until his defeat, Dark Riku tricks everyone he meets to believe that he is in fact Riku during the time he was possessed by Ansem during Kingdom Hearts, pridefully referring to that time as his finest hour when talking with the real Riku.

Outside of these traits, Dark Riku is shown to dislike Organization missions involving conducting experiments, merely calling them a “hassle” and showing a more preferable taste for combat.

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 and retained his original aquamarine color. It should be noted that Dark Riku’s design and overall model in his debut appearance is completely identical to Data-Riku from Kingdom Hearts coded.

Like a majority of the Thirteen Seekers of Darkness, Dark Riku primarily appears wearing a black coat. During the Keyblade War, Dark Riku uses Dark Mode and the bodysuit given to him.

Kingdom Hearts 3D: Dream Drop Distance
Having had his heart collected in the past by Young Xehanort, the Riku Replica is brought to the present to serve as one of Thirteen Seekers of Darkness. 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 reminding him of his past misdeeds and weakness in having surrendered to the darkness. Ending up in Prankster's Paradise, Dark Riku lures Pinocchio away from Geppetto, evoking déjà vu for the visiting Riku, who soon ventures out to find him. Riku and Jiminy Cricket soon encounter the hooded Dark Riku with Pinocchio in the depths of Monstro before demanding him to release Pinocchio. Silently complying, Dark Riku allows Pinocchio to reunite with Jiminy before 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, leaving Riku to lament over his past interactions with the darkness within him.

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.

Kingdom Hearts III
With Master Xehanort fully resurrected, he infuses parts of his heart among his vessels, giving them all his signature golden eyes to signify their affiliation to him, including Dark Riku. While conducting an experiment, Dark Riku appears before Sora and Big Hero 6 in San Fransokyo, using the Darkubes 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 since their encounter in Monstro. 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, calling it to be a "real test for the Mark of Mastery". When Riku asks what he had just said, Xigbar appears, telling the guardians that they have to prove themselves one last time after their previous blunder before ordering Dark Riku to "take it away". After Xigbar leaves, Dark Riku unleashes a surge of darkness, releasing a massive swarm of Heartless from within the effigy before warping away in a mist of darkness. 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 newer replicas created by Vexen. The present version of the Riku Replica emerges from within Riku's heart, and forces Dark Riku's heart 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, making him the first member of the thirteen darknesses to fall.

Following the defeat of Master Xehanort at Scala ad Caelum, Dark Riku's empty replica body was delivered to Radiant Garden, where it was used to house Naminé's heart.

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, some of his abilities are identical to Ansem's, albeit color-swapped into a purple color as opposed to Ansem's dark red. These attacks consist of Dark Riku summoning a half ring of purple crystals in front of him, which will explode if anything comes into contact with them, as well as summoning a pool of darkness that homes below his target for a few moments before manifesting into a larger pool of darkness that unleashes multiple dark claws that will harm his target if they remain within the pool.

Like his original self as the Riku Replica, Dark Riku will perform a three swing combo, the last one unleashing a V-shaped dark shockwave on the ground. He is also capable of sending out a slash of energy from his Soul Eater from a distance before rushing his target for a single slash. At times in the battle, Dark Riku will use his desperation move, Helm Split; Dark Riku will rise into the air in a dark aura before warping above and slamming down onto his target, warping through the ground and repeating his slam attack multiple times before unleashing a swarm of lightning-based spires from the ground.

Weapons
In conjunction with his powers, Dark Riku wields the Soul Eater, which powers up with the use of darkness.