Riku is one of several original characters appearing in Kingdom Hearts Birth by Sleep, Kingdom Hearts, Kingdom Hearts: Chain of Memories, Kingdom Hearts 358/2 Days, Kingdom Hearts II, and also one of the main characters in the series. Riku lives on Destiny Islands. He is the best friend of Sora and Kairi. Riku is five years old during Kingdom Hearts Birth by Sleep, fifteen during the events of Kingdom Hearts and Kingdom Hearts: Chain of Memories, and sixteen during the events of Kingdom Hearts 358/2 Days and Kingdom Hearts II. Riku’s name is derived from the Japanese word for “Land”.
Riku appears as a cool and collected 15-year-old. He and his best friends Sora and Kairi live on the Destiny Islands. The three spend their days constructing a raft and gathering supplies, so that they might one day set out and search for other worlds. He and Sora share a fierce but good natured rivalry, competing over who is stronger in battle, among other things. Despite the fact that their arguments are of good intention, Riku is known to irritate Sora by being a bit of a sore loser and (in the original Kingdom Hearts manga, Volume 1) very pushy involving his fight over Kairi, shown when he tells Sora that, if the latter does not share a paopu fruit with Kairi soon, he will. All three of them, as well as the other children that live on the islands, spend most of their time on the smallest island, acting as their own personal playground.
However, things take a drastic turn on the night before their journey. Riku, likely unable to leave the islands without knowing the truth, opens the door in the secret place and the Heartless, beings of darkness, invade the islands, seeking out the world’s heart. Both afraid that a storm would destroy the raft, Kairi and Sora set out for the island. Sora finds Riku first. However, while Sora is concerned for his friend’s safety, Riku is only obsessed with finally escaping the islands. Darkness surrounds the two, and Riku disappears into it, while Sora gains the Keyblade.
Riku eventually ends up in Hollow Bastion via the Corridor of Darkness, separated from his friends. There, he is found by Maleficent and she offered shelter and protection.
Though Riku is provided a place to stay, Maleficent begins corrupting his mind, convincing him that Sora has replaced him and Kairi with Donald Duck and Goofy. She claims that the only way to save Kairi, who has lost her heart to the darkness, is to ally himself with her and her band of villains, while abandoning Sora. Not long after, he meets Sora in Traverse Town, takes a look at his Keyblade, holds it for a few moments, and gives it back to Sora without it disappearing from his hand – a foreshadowing of Riku’s role as a Keyblade wielder. After his reunion with Sora in Traverse Town, Riku begins to agree with Maleficent and officially starts his stay with her. However, he is still highly suspicious of her motives.
As the days progress, Riku becomes steadily angrier at Sora, convincing himself to hate his old friend. He obtains a blade from Maleficent called the Soul Eater and teaches himself to use it, believing it will aid him in his search for Kairi’s heart.
With his mind corrupted, Riku meets Sora once again in Monstro, intending to use Pinocchio to help him find a way to save Kairi’s heart. However, Pinocchio ends up captured by a giant Heartless called the Parasite Cage, and Riku is forced (somewhat against his own wishes) to work together with Sora, Donald, and Goofy to save him. Afterwards, Riku kidnaps Pinocchio, and is pursued by Sora, Donald, and Goofy into Monstro’s stomach. Riku asks Sora to join him and help him rescue Kairi, but Sora refuses, knowing that Riku’s methods are wrong. When the Parasite Cage returns, Riku vanishes into a Corridor of Darkness.
Riku later appears at Neverland, traveling on Captain Hook‘s ship, where Kairi’s body is held. Riku also now shows that he has gained the power to control the Heartless, a gift granted to him by Maleficent. He then goes to Sora, revealing that he has Kairi’s body in his possession. Sora warns Riku that the Heartless will eventually turn on him and devour his heart, but Riku refuses to listen and retreats after creating AntiSora, a dark reflection of Sora, with his newly acquired dark powers. However, before he heads back to Hollow Bastion, he instructs Captain Hook that the captured Wendy is not a Princess of Heart. Riku takes Kairi and recklessly returns to Maleficent’s castle via the Corridors of Darkness without a vessel, a foolish act that strains even his powerful heart. While he is drained from the trip, Maleficent unlocks the last of Riku’s power and the boy’s heart is plunged into almost complete darkness.
At Hollow Bastion, Sora again meets Riku, and defends the wounded Beast from a swift death by Riku’s blade. But the Beast is hardly one of Riku’s major concerns, as Riku reveals his true purpose for confronting Sora. He is able to wield the Keyblade. Riku explains that Sora was only a temporary recipient, telling him that he was “just the delivery boy” and that Riku is the true Keyblade master. After leaving Sora with the toy sword he used on Destiny Islands, he departs, and Donald and Goofy follow him as per The King‘s orders, though regretfully.
Though Sora is significantly weakened, he makes his way to the foyer of the castle; with the help of Beast. Riku, having taken on a new appearance with the increasing strength of his dark powers, tells him to quit, but Sora refuses to leave without Kairi. Riku, intending to prove to Sora that he is the strongest of the two, attacks his old friend, only for Donald and Goofy to go to his aid after finding themselves unable to betray Sora after all that they had been through together. With the strength of his friends, Sora regains the Keyblade, becoming the true master, and drives Riku to fight him.
Eventually, Riku manages to escape, but is confused as to why Sora was able to regain the Keyblade. A mysterious voice somehow contacts him from the Darkness, telling him that only one with a stronger heart can claim the Keyblade, and Riku himself was weaker than Sora. However, the mysterious voice, now slowly taking a shape, presents Riku with an offer he can not resist, a Keyblade of his own. Under the influence of jealousy and rage, Riku agrees to become one with the Darkness, only to become possessed by the mysterious voice itself.
The new Riku then presents himself to Maleficent and warns her of the dangers of unlocking the Final Keyhole. When she claims not to be afraid, Riku shows a new, dark Keyblade created with six of the seven princesses’ hearts. However, it doesn’t react to the Keyhole. Realizing it is still needing Kairi’s heart to be complete, Maleficent heads off to handle Sora and co. After Maleficent is defeated by Sora, Riku appears with his Keyblade and explains that unlike Sora’s Keyblade, this Keyblade possesses the ability to unlock a person’s heart, and Riku demonstrates this by piercing Maleficent’s chest, allowing the immense darkness that is attracted to her to overtake her heart, transforming her into a giant dragon. While this form proved a far deadlier enemy to Sora and his friends, it still was defeated and killed.
Riku, calling himself Ansem, the Seeker of Darkness, later confronts and battles Sora (who is cut off from Donald and Goofy) at the Final Keyhole. He hopes to release Kairi’s heart from within Sora’s, so that she, being the final Princess of Heart, can open the Keyhole. He fails, but Sora impales himself with the Keyblade anyway to save Kairi. With Sora gone, Ansem attacks Kairi, Donald, and Goofy, but they are saved by Riku, who is using the last of his strength to hold the dark powers of his possessor off.
Riku’s rejection of Ansem leads to an internal struggle between the two, which Riku ultimately loses. This causes Riku to lose control of his body once more, while Ansem gains it fully.
A newly restored Sora, along with Donald and Goofy, eventually make their way to the End of the World, a world made of the worlds that have been destroyed by the Heartless. There they once again confront Ansem and defeat him after a series of battles at the Door to Darkness.
Riku is then restored, and appears on the other side of the Door to Darkness, aiding Sora in closing it, but unable to cross it, as only pure darkness can go back and forth through the door. Waiting to enter the Realm of Light are thousands upon thousands of Heartless, which only causes further problems for the two friends. However, King Mickey, appearing for the first time in the game, wards them off with the powerful Kingdom Key D. The King and Sora then proceed to close the door, Sora on the outside, Mickey within. Riku is stuck on the other side of the door with the King, but not before he told Sora to “take care of her,” meaning Kairi.