Riku and Kairi had been childhood friends along with Sora. Kairi showed up on Destiny Islands one night following a meteor shower. She was adopted by the mayor and Riku suggested that he and Sora go to see her. After Kairi became friends with Riku and Sora. the two boys seemed to develop a crush on her at some point. This caused Riku to get a little competitive with Sora as he was slightly jealous of him getting Kairi's attention at some points. Since Riku is a little older than Sora, he was often superior to him in their battles and the other islanders assumed that Kairi could always count on him.
Riku and Kairi eventually decided to travel to other worlds along with Sora. They worked on building a raft together and intended to use it for their travels. At one point, Riku was about to race Sora and decided that the winner would get to share a Paopu fruit with Kairi, although it turned out that he was just kidding. The heartless eventually attacked Destiny Islands and Riku willingly allowed the darkness to take him from the world. Sora voiced his concern about Kairi but Riku was certain that she would also make it off of the island.
Riku ended up in Hollow Bastion where he met Maleficent. He worked for her and in return she helped him find Kairi. It turned out that she lost her heart and was stuck in a comatose state. Riku was determined to help Kairi get her body back so that she could wake up. He attempted to capture Pinocchio and use his heart to return Kairi's but was stopped by Sora. When Riku encountered Sora aboard Captain Hook's ship, he revealed that he found Kairi. When Sora caught up to him, he used a corridor of darkness to take her back to Hollow Bastion.
Riku was eventually possessed by Ansem, the Seeker of Darkness. While possessed, he revealed to Sora that she was missing her heart and found that it was inside of Sora's heart. Sora defeated him and released Kairi's heart which allowed her to wake up. Ansem then appeared in full form and prepared to destroy Kairi until Riku held him back. He and Sora eventually helped each other close the Door to Darkness and Riku told him to take care of Kairi. He eventually found Sora sleeping in a pod in Castle Oblivion and complained that he was taking a nap instead of taking care of Kairi.
Kairi spent over a year on Destiny Islands waiting for Sora and Riku to return but was eventually captured by Organization XIII. Naminé showed up to rescue her until Riku appeared to help her. Kairi took his hood down and noticed his altered appearance. When they were reunited with Sora, she revealed his identity and helped Sora see that he was actually Riku. They were separated during the final battle with Xemnas but Riku was eventually returned to Destiny Islands along with Sora and Kairi happily greeted them. Riku was eventually called to take the Mark of Mastery exam and was named a keyblade master. Yen Sid then had him bring Kairi to him because it turned out that she had the ability to wield a keyblade.
RiKai is a rather popular ship in the Kingdom Hearts fandom. It is not as popular as SoKai or Soriku but is still has a decent following. It was obvious to most fans that Riku had a crush on Kairi and that this was the main reason for him being jealous of Sora. RiKai is commonly shipped by fans who prefer to ship Sora with someone else such as Naminé.
- It does seem that Riku had a crush on Kairi and was initially jealous of Sora because of this but eventually accepted the fact that she wanted to be with Sora.