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
As one of the few time-traveling members of the Organization, Dark Riku was given a new refined replica body to use as a vessel, and was later given a fragment of Master Xehanort's heart like the rest of the Organization.
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, acting incredibly smug and egotistical.
He is also shown to be extremely cruel, laughing at Hiro's pleas to release the original Baymax while treating the latter as nothing more than a puppet.
For all of Kingdom Hearts III up until his defeat, Dark Riku fools everyone he meets into believing that he is in fact Riku during the time he was possessed by Ansem in Kingdom Hearts, 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 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 Riku wearing his black coat at the end of Kingdom Hearts: Chain of Memories, and Data-Riku from Kingdom Hearts coded.
Having had his heart collected in the past by Young Xehanort, the Riku Replica is brought to the present to serve as a member of the new Organization XIII. Given a new replica body to inhabit, the newly dubbed Dark Riku is 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.
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 members of the real Organization at Where Nothing Gathers before he and the others are forced to temporarily withdraw due to their allotted time running out.
Dark Riku, now with golden eyes, appears before Sora and Big Hero 6 in San Fransokyo, claiming to be Riku's past self while he was possessed by Ansem. He reveals to the group that he is conducting an experiment with the Darkubes to gather data on negative emotions, with the intent of seeing if a heart could be recreated from data, a concept the Organization stole from Sora's attempt to revive Roxas.
Using the original Baymax's combat chip, Dark Riku uses the Darkubes to imbue it with negative data they collected from the hearts of citizens from the city, creating a digital heart born of conflict from the chip. Retrieving the original Baymax's body from the alternate dimension it was lost in, Dark Riku combines the Darkubes and the combat chip with it, creating Dark Baymax before taking his leave from the world.
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 Organization 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 refined 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 real Organization to be defeated.
Once the battle concludes and Sora leaves to continue fighting other Organization members, Dark Riku's empty replica body is collected by Demyx and brought to Radiant Garden, where it would be 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, harming his target if they remain within the pool.
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 weapon from a distance before rushing his target for a single slash. At times in 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 back into the air and repeating his slam attack multiple times before unleashing a swarm of lightning-based spires from the ground.
In conjunction with his powers, Dark Riku wields the Soul Eater, which powers up with the use of darkness.