Riku was one of the main characters in the Kingdom Hearts stories. He was Sora's best friend and also the true and original bearer of the Keyblade itself, making him much more than meets the eye. The only reason that he didn't ever receive the original blade was because he was willing to let his heart fall into darkness in order to save his friends. The Keyblade would only accept a bearer who's heart was pure and clean, so instead gave it to Sora, who had been the nearest person before Riku gave himself into darkness to save Kairi. Like Sora, he had an affectionate place in his heart for Kairi, but unlike Sora, when he wanted something he did his best to get it. He was somewhat Sora's rival, but at the same time his friend and enemy. On Destiny Island, he proves that he has much more confidence in himself than Sora and generally does things better than him. When he becomes evil in order to get Kairi back, he allows himself to become far more corrupt than he had planned, and ends up becoming a vassel for Xehanort, or at least Xehanort's heartless that called himself Ansem. In Chain of Memories, he goes downward in Castle Oblivion, spiraling into the base of the castle, and in the end finds his memories again. After growing older in Kingdom Herarts II, he loses his original form to the appearance of Xehanort's heartless, and is forced to look like it until the end of his journey when a strange machine turns him back to his older and original self. He finds light in the darkness he wove for himself, and continued onward to assist Sora in his plight for the final battle against the nobodies.
At the age of 15, Riku and Sora lived on Destiny Island along with some other friends. When a girl named Kairi came to the island, Riku got the idea to build a raft to see the other worlds out there and possibly find the one Kairi came from. The three of them became good friends and work hard on finishing the raft. One day, Riku and Sora were going to race to see who gets to name the raft. Riku then said right before they started that the winner also gets to give the palpu fruit to Kairi. After the race Sora seemed angry but Riku said he was just joking. Then that night there was a storm and when Sora arrived he saw Riku standing in the darkness. When the darkness went to consume him, he didn't fight back and let him take him hoping it would take him to a new world.
When Riku arrived to his new location he was at the Hollow Bastion, where he met Maleficent. She convinced Riku that Sora had forgotten about him and Kairi and only cared about his new friends, Donald and Goofy.
When Sora is swallowed by a whale, Riku is there trying to capture Pinocchio. He said that Kairi needed the puppets heart and that the puppet didn't need it. Sora thought this was wrong, even if it was to help Kairi though. Riku then left Sora to battle a heartless while he escaped.
When Sora was captured on Captain Hook's ship, Riku appeared to being running things. He also showed Sora that he had found Kairi and she really has lost her heart to the Heartless. Before Sora could get to Kairi to save her, Riku hand taken her to Hallow Bastion where the other 7 princesses of destiny were being gathered. Sora arrived at Hallow Bastion who found Riku battling Beast(Beauty and the Beast).
Beast had come to find Belle who was captured with the other princesses. After defeating Beast, Riku told Sora that his heart wasn't strong enough for the Keyblade and was able to take it from him. He then told him to go play hero and through him a toy sword. Donald and Goofy then had to follow Riku, cause King Mickey told them to follow the "Key".
Hallow Bastion and the End of the World
With the help of Beast, Sora made his way into the castle where he confronted Riku against. Riku fired a blast of darkness at Sora which was block be Goofy. Goofy told Donald to tell King Mickey he was sorry, but Donald then joined Goofy and Sora. With his friends Sora was convinced that his heart is strong because of his friends which allowed him to take back the Keyblade. The three of them battled against Riku who lost, and made his escape. While escaping, Ansem came to Riku and took over his body.
When Maleficent lost to Sora and his friends she came to Riku who used his dark Keyblade to transform Maleficent into a black dragon. Even more powerful, she battled Sora again but still could not defeat him. Riku went into the Door to Darkness. Sora followed him to a world created from the remains of worlds. After making his way through it appeared that Sora had arrived back on Destiny Island. On the beach they saw Riku who revealed himself to be Ansem. He told them that Riku was too lost in the darkness to be saved.
After Sora defeated Ansem, Sora, Donald, and Goofy were trying to seal the door to Kingdom Hearts. When they looked inside they saw thousands of Heartless. Riku suddenly appeared on the other side and was trying to help them close the door from the other side. King Mickey also appeared on the same side as Riku. Together they were able to seal the door but Mickey and Riku were locked on the other side.
Chain of Memories
Riku awakens in Castle Oblivion but he is in the basement. He then works his way through the Castle to the top. While on his way he meets a man name DiZ. When Riku finally arrives at the top, he sees a girl named Naminé who is attempting to restore Sora's memories. Riku then decides that he needs to try to find a balance between the Darkness and Light in him. Later, Riku battles Roxas(Sora's Nobody) in an attempt by DiZ to restore Sora's memories. While battling Roxas, Riku relies on the darkness and is transformed into a form that looks like Ansem(Which is later revealed to be Xehanort's Heartless and not Ansem at all). When Riku returned to DiZ, DiZ asked who he was and Riku replied, "Ansem". Riku then tries to help Sora through his journey but never revealing himself to anyone. He didn't want anyone to see what he had become.
The World That Never Was
When Kairi and Naminé were captured by the Organization XIII, Riku arrived in The World That Never Was to save them. After setting them free he was about to run off into a darkness portal, when Naminé asked Riku to stay. When Kairi hear Naminé calling him Riku she wanted to see if it was true. She walked up to him and removed his hood only to see Xehanort's Heartless face. Even with this new form, Kairi could tell it was Riku. The two of them then went to help Sora. Sora then told Riku thank you for saving Kairi, though he was thinking it was Xehanort's Heartless. Kairi told Sora it was Riku which Sora soon believed himself. Riku then helped them make their way through castle. He was finally turned back to normal when DiZ(now revealed to be the real Ansem) machine, to turn Kingdom Hearts into data, exploded. Riku and Sora then battled Xehanort's Nobody. After defeating him the two found a portal which took them to a beach in the World of Darkness. They both seemed like they could be fine there if that's all that the world was. But when a letter Kairi wrote to Sora floated to them in a bottle, the Door to Light opened up which led the two back to their Island. Soon, Sora received a new letter from Mickey which has a possible request for the three friends. Riku helps sora anyway he can but from the darkness.
Later on Riku passes his Mark of Mastery test and becomes a True Keyblade Master.