It is unknown exactly when Riku learns about Roxas, but it likely happens while he is working for DiZ. While Riku is in Castle Oblivion, he learns that Sora is in a coma while having his memories restored. He meets Xion who is the replica of Roxas and learns more about the organization. Roxas eventually leaves the organization and tries to take them on once Xion fades away. Riku confronts him in a battle but is unable to defeat him. Feeling that he has no other choice, he gives into the darkness and is able to subdue Roxas after taking on the appearance of Xehanort's heartless.
With his memories fading, Riku brings Roxas back to Ansem the Wise who temporarily erases his memories and places him into a simulated Twilight Town. While Roxas is waiting to have his memories restored, Riku is tasked with watching over him and making sure that he is safe. Roxas is affected by Sora regaining his memories and sees Riku in his dreams. Riku is forced to steal a munny pouch and a blue crystal from Roxas but also saves him from falling when he falls off of a clock tower. He also eventually helps protect the mansion from nobodies while Roxas enters to eventually go and fuse with Sora.
Sora is finally able to wake up after Roxas joins with him. Riku briefly encounters Sora in the Realm of Darkness and gives him a picture that includes Roxas. Sora is eventually pulled into his Station of Awakening to fight Roxas. Sora asks if he is Riku and Roxas responding that he once defeated Riku. After Sora and Riku are reunited, the organization members keep referring to the former as Roxas. Sora is confused and Riku finally explains that Roxas is his nobody and everything that happened. Sora feels sad and wants to meet him so Riku reminds Sora that Roxas is already in his heart.
Sora and Riku later travel through the Realm of Sleep while taking the Mark of Mastery exam. Sora is eventually stuck in a coma again and Riku is forced to go save him. While inside of Sora's subconscious, Roxas appears and asks what Riku is afraid of. Once Riku answers, Roxas disappears and Ventus appears shortly after. Riku initially mistakes him for Roxas but quickly realizes that they are not the same person.
RikuRoku has small amount of popularity in the Kingdom Hearts fandom. It is not as popular as AkuRoku, Soriku or het pairings involving Roxas but it still has its fans. A lot of fans shipped in Kingdom Hearts II when it was revealed that Riku was the one watching over Roxas. Riku also showed pity on Roxas when he learned that he would have to give up his life so that Sora could wake up.