Riku and Naminé go back since Kingdom Hearts: Chain of Memories. Unlike Sora who had his memories being erased, Riku still had the darkness lurking within him. The silver haired boy first hears about Naminé when he visited Twilight Town. DiZ tells him to go see her, so he tries to climb higher beyond the castle basement to find her.

His first interaction with Naminé was actually when Zexion tried to destroy Riku, but instead of just choosing light, Kairi (who was actually Naminé transformed into Kairi) tells him that darkness and light is what makes Riku strong, so he should choose both powers. This made Riku stronger.

When Riku finally reached the floor where Naminé resided, she asked him a question. It was about whether he’d choose to lock away his darkness or keep it. In the end Riku keeps the darkness and will try his hardest to fight away Ansem. His last words to her were to take care of Sora seeing how he was in the pod.

In Kingdom Hearts II, Riku and Naminé seem to have befriended each other, but Riku sides with DiZ knowing if she told the truth to Roxas about his origin, it would ruin DiZ's plan. At the ending, they don't interact at all but when she finished fusing with Kairi along with Roxas to Sora, Riku tells them they're still the same, and they're now all complete.