I'll show you how powerful I am!
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 for his heart, and would additionally be infused with a fragment of Master Xehanort's heart alongside the rest of his fellow Organization members.
As he is in reality the Riku Replica brainwashed into believing he is the real Riku, Dark Riku's personality stems from Riku's in Kingdom Hearts. He harbors an intense competitive streak, wishing to be stronger and better in every way possible. Like the Riku Replica, he shows no fear in using the darkness, pridefully boasting of its vast powerful applications and the superior strength he gained from using it.
His competitive nature and desire for power however causes Dark Riku to be incredibly cocky, arrogant, and egotistical, commonly talking down to those he sees as weak, including the very person he is derived from. For a time, Dark Riku maintained a lasting charade that he was in fact Riku during the time he was possessed by Ansem, Seeker of Darkness, though this would ultimately falter upon his defeat.
Outside of these traits, Dark Riku is shown to have a slight disdain for the Organization work he was tasked with conducting in San Fransokyo, merely calling it a "hassle" and showing a liking for a more brawn approach to his work.
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 from the past by Young Xehanort, the Riku Replica is brought to the present to serve as a member of the real Organization XIII, though he is forced to appear as a corporeal heart for the time being without a proper vessel. Now going under the alias of “Dark Riku”, he is soon tasked along with Ansem 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 to welcome the return of Master Xehanort and to induct Sora as their final member, though their efforts to convert Sora are cut short, forcing the incomplete Organization to withdraw.
With their time up as corporeal hearts, Dark Riku and the other time-traveling members of the Organization are forced to return to their original times, though they soon regather in the future once more, this time with refined replicas from the early batches of the Replica Program waiting for them to use as vessels for their hearts, thus allowing them to remain anchored to the present. In addition, Dark Riku and the rest of the Organization are given a fragment of Xehanort's heart, gaining golden eyes in the process.
Dark Riku appears before Sora's party 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 the first 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 and Nobodies 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 guardians of light at the crossroads of the Keyblade Graveyard, where Master Xehanort would create the Skein of Severance to serve as the battlegrounds of the Keyblade War. Dark Riku, having returned to Dark Mode, soon regroups with Ansem and Xigbar in the Trail of Valediction, where they would fight Riku in battle. 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-day 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 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 eliminated.
Once the battle concludes and Sora leaves to continue fighting in the Keyblade War, 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 and ultimately bring her back to life.
One year after Master Xehanort's defeat and Sora's disappearance, a data recreation of Dark Riku, along with the rest of the real Organization XIII, is fought by a digital copy of Sora with the hopes of uncovering the real Sora's location from the encrypted data they possessed. After defeating all thirteen data recreations however, the results proved inconclusive.
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. Outside of Dark Mode, Dark Riku displays the ability to use corridors of darkness to traverse the three realms at will, and is shown to have the power to open portals to other dimensions outside of the three realms, such as the alternate dimension where the original Baymax was lost in. He is also shown to be familiar with manipulating technology, melding this skill with dark magic to control the Darkubes and to conduct his experiment of creating a digital heart.
In combat, Dark Riku’s basic attacks mirror that of his various other incarnations, such as his three swing combo that unleashes 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, appearing to be a combination of Dark Aura and 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 several times before unleashing a swarm of lightning-based spires from the ground on the final slam.
It should be noted that 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 crystal mines 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.
In Kingdom Hearts III Re Mind, a digital recreation of Dark Riku is fought in the Data Analysis, and is canonically his true power level. Easily one of the most aggressive data fights along with Saïx, Dark Riku will relentlessly attack Sora with amped-up attacks seen during his fight in the Keyblade Graveyard, and will mix in unblockable attacks in conjunction with his basic attacks, such as turning the slash of energy he can release from his blade into an unblockable attack. During the fight, Dark Riku will summon massive quantities of his dark mines all around the arena, utilize Dark Reflect while sidestepping, constantly slam down onto Sora while simultaneously creating unblockable spires of lightning all around Sora, launch more pools of darkness, and unleash powerful shockwaves from his combos in greater sizes than previously displayed.
His most devastating attack is his desperation move Dark Aura, which he can use as early as his first health bar in the fight. Dark Riku will start the attack by raising into the air in an aura of darkness, slamming down onto the center of the arena and creating a shockwave of darkness before warping towards Sora from all directions. While he is doing this, he will summon formations of his dark mines all around the arena and unblockable lightning spires each time he finishes a warp attack on Sora, and will also directly crash down onto Sora, additionally also slamming down into the center of the arena to create more shockwaves as shown at the start of his onslaught. At the end of it all, Dark Riku will close the desperation move by summoning his unblockable lightning spires across the entire arena.
In conjunction with his powers, Dark Riku wields the Soul Eater, which powers up with the use of darkness. It is a falchion-esque sword, with a red, purple and blue blade shaped like a demonic wing. Its handle bears a blue eye, and the handle itself consists of dark braids.