Riku and Xion first met outside of the Beast's Castle. Riku had been wearing the same type of robe as Organization XIII so they considered him an "imposter" and assigned Xion with the task of defeating him. However, Riku won the fight and called Xion's keyblade a sham. He was upset about the fact that she even had one. He briefly lifted up his blindfold to look at her and saw Sora.
Xion eventually leaves the organization and joins up with Riku. They went to Destiny Islands together and Xion falls into an intense nightmare flooded with memories. Riku comforts her through this process. He then explains Sora and Kairi to her when she asks about them. Xion learns that she must give up her existence if Sora is to wake up but Riku leaves her alone and allows her to decide for herself.
Xion eventually goes to Castle Oblivion with Riku. Roxas and Axel encounter them in Twilight Town but Riku distracts them long enough for Xion to get away from them. Xion is eventually absorbed by Roxas but he is upset by this and is determined to bring her back. Riku fights Roxas and points out that it is difficult to even remember her name. Roxas summons both of his keyblades but Xion makes him pass one of them to Riku.
As soon as Riku catches one of Roxas' keyblades, memories of Xion instantly appear in his head. He realizes that Xion wants him to stop Roxas from taking on the organization and getting himself killed. Riku has difficulty defeating Roxas until Xion contacts him telepathically and begs him to stop him. Riku then allows Xehanort's heartless to take over his form so that he can incapacitate Roxas. Riku eventually forgot about Xion as he met her inside of Sora's heart and did not remember her. She asked him what he wishes for. Riku answers this question and this allows Sora to wake up.
Rion is a very popular Kingdom Hearts ship. It might not be as popular as RokuShi or Soriku but it is still very popular. It mostly became popular after the events of Kingdom Hearts 358/2 Days. By that point, Kairi seemed to be the love interest of Sora while Naminé appeared to be the love interest of Roxas. Many fans began to pair Xion with Riku as this resolved the issue of them not having a love interest. While the two briefly started as enemies, they quickly became allies and many fans shipped them together.
- In the manga, Riku sees Xion as Kairi.