Dark Riku (ダークリク Dāku Riku), is the name given to the alternate form of Riku when he is possessed by Xehanort's Heartless, entering his Dark Mode in KINGDOM HEARTS. He was first seen in McLeodGaming's flash cartoon series, A Super Mario World.
Dark Mode is a special form Riku can transform into in KINGDOM HEARTS and KINGDOM HEARTS: Chain of Memories. Riku initially receives this ability from Maleficent, and uses it against Sora after Sora reclaims the Keyblade, but loses it after his body is taken by Ansem.
After entering Castle Oblivion, Ansem manipulates the darkness in Riku's heart, giving Riku the ability to use Dark Mode freely. In this form, Riku can use his darkness without the worry of being taken over by Ansem. Because Riku is so darkness-oriented, this form gives him a substantial advantage in battle, such as increased speed and feeding the Soul Eater's unique power-up ability with the power of Darkness. Riku Replica can embrace the darkness willingly, thus he can also use and stay in Dark Mode.
In A Super Mario World
Dark Riku, simply referred to as Riku, appears as one of the antagonists in the final episode of A Super Mario World where he struggles in a fight with Cloud Strife. He ultimately defeated when he and Cloud deliver their final blows at the same time, but only Riku is affected by it, and crumbles into bloody pieces.