Riku is Sora's and Kairi's best friend and also a Keyblade wielder.
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.
In Chain of Memories, 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 Ansem the Wise in his DiZ guise, 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 remains of Ansem, Seeker of Darkness within him. 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 to capture him at the cost of becoming a physical manifestation of Ansem, Seeker of Darkness.
Trapped in that form, Riku helps Sora in secret, ashamed of his appearance and gaining the Way to the Dawn 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 and remained there for a year before Sora receives a letter from Mickey explaining the fates of Terra, Ventus, and Aqua. Now teaming up with Sora, Riku goes on a quest to complete the Mark of Mastery to become a Keyblade Master in preparation for his eventual battle against Master Xehanort.
Riku appeared in most of the Ultima which also he helped the Ultimawielder several time in their journey finding the weapon and also other dangerous artifact.
Riku also star in major role in Dimensional Empire, as he and Sora about take the mark of mastery test, their route has been cut short by the sudden wormhole and scattered to an unknown new dimension