Riku (リク?) is Sora's and Kairi's best friend, and character created by Tetsuya Nomura for the Kingdom Hearts series.[1] Nomura has stated that Riku's name can be interpreted as "land";[18] the Japanese on'yomi pronunciation of land is riku (陸?).[17][19] Riku is voiced by Mamoru Miyano in the Japanese version and by David Gallagher in the English version. As a child in Kingdom Hearts Birth by Sleep, he is voiced by Eiji Shima in the Japanese version and Ty Panitz in the English version. He is playable in the second story mode of Kingdom Hearts: Chain of Memories and as a world-specific ally in Kingdom Hearts II. In 358/2 Days, Riku is playable in the multiplayer mode and also serves as the final boss in the game's story mode. He also acts as Sora's main rival, both in combat and winning the affections of Kairi. The latter, however, might be just Riku playfully provoking Sora, since he and Kairi just appear to have a brotherly/sisterly relationship.

As a young boy in Birth by Sleep, Riku becomes curious about going to other worlds and gaining the strength needed to protect his friends, which leads him to being chosen by Terra to inherit the power of the Keyblade. By the beginning of the main series, now fifteen years old, Riku goes with the darkness as their world is attacked by the Heartless. Separated from his friends, Riku ends up at Hollow Bastion where Maleficent finds him, thus giving him the Soul Eater. Under her influence, Riku turns on Sora and searches for a way to help Kairi.[17] Riku eventually becomes possessed by Xehanort's Heartless, with his heart sent to the realm of darkness where he meets King Mickey and helps him and Sora seal the door to the realm of darkness.

In Chain of Memories, through DiZ, Riku awakens in the basement levels of Castle Oblivion and begins climbing to the ground floor. Along the way, Riku confronts the remaining darkness in his heart and a replica of himself that is created by Vexen; he also encounters DiZ, who has been overseeing his progress. After being reunited with Mickey, Riku reaches the main floor where he finds Naminé as she attends to a sleeping Sora. Riku chooses to find a balance between his light and darkness, allowing him to defeat what remained of Xehanort's Heartless. Soon after, Riku joins DiZ to ensure Sora is restored in peace.

While capturing Roxas in order to make Sora complete again, Riku uses the power of darkness at the cost of becoming a physical manifestation of Xehanort's Heartless.[22] Trapped in that form, Riku helps Sora in secret, ashamed of his appearance and gaining the Way to the Dawn (ウェイトゥザドーン?) Keyblade. Riku returns to his original form when Ansem the Wise's machine to digitally seal away Kingdom Hearts malfunctions and self-destructs. Joining Sora, they battle and defeat Xemnas. In the end, he returns to Destiny Islands.