Riku is a survivor of the Destruction of Destiny Islands and a chosen wielder of the keyblade. But his own story is a bit different than Sora's. When he was separated, Maleficent tricked him into working for him when he saw Sora presumably moving on from him. He worked on collecting the Seven Princesses of Light for Maleficent in exchange for finding Kairi. After a loss to Sora and the group, he was approached by Ansem, Seeker of Darkness and had his body taken over by him. After being freed from the hold, Riku appeared on the other side of the Door to Darkness and attempted to close it on his own. Which would have been futile if it weren’t for Hope and his groups assistance.
After that, they walked through the Realm of Darkness for a long time before somehow waking up in the basement floors of Castle Oblivion. Recapping Jexi’s previous adventures, Riku faced members of Organization XIII Vexen, Lexaeus and Zexion. That was when he learned to finally embrace the darkness and use it to conquer his inner demons. After one last clash with Ansem, Riku and the heroes went their separate way. When Hope asked if he was gonna walk a road to nightfall, Riku replied “No. Its the Road to Dawn.”
Over a year, Riku worked for Diz, restoring Sora’s memories and watching the Organization. When it finally came to it, Riku and the rest of them engaged in an extermination of the Organization’s remaining members. Together, they beat Xemnas and stopped him from obtaining Kingdom Hearts. He also had a part in stopping the Ouma crisis.
Today, he now takes his Mark of Mastery Test in the Dream Realm, unlocking seven sleeping keyholes. But after that, he would know his true place. Its not where he needs to go next, it’s where he needs to go back to. He would become the second Keyblade Master on the Squadron.