I have played all the games - many times. Reread what I posted. I was talking about his JOURNEY. During Riku's journey from KH1-3, Sora's barely been around him. Riku didn't give a crap about Sora in KH1 until the end when he realized what was going on, then Riku began his quest for redemption. Yes, he was trying to redeem himself for Sora... but Sora himself wasn't along that road to guide Riku to that redemption (at least not for most of it). The reason why Riku was even able to face Sora in KH2 is because of what he learned from Ansem, Mickey, and Namine (Riku Replica, Roxas, and Xion can be thrown in there as well). So yes, Sora matters the most to Riku, but I don't see the problem with Ansem, Mickey, and Namine being there as they have always had a major influence on guiding Riku into the person (in his mind) worthy of being Sora's friend.
Also, and this is just my personal preference, I kind of like showcasing that character's have connections OUTSIDE of Sora. When I saw Riku's station in KH3, I was automatically expecting Sora, but when I saw Ansem/Mickey/Namine I was reminded how much they actually influenced him and how much story they had outside of Sora. In a game that was focused on everything coming back to Sora, so much so that certain character interactions had to be sacrificed in the name of Sora being present, I think it's refreshing to note that characters actually can do stuff beyond interacting with Sora. Riku being a prime example.
Here's all the cutscenes from KH1, the most important game in setting up Riku. If you don't want to watch them fine, let me ask you something, what pushed Riku to meet all those people? What is Riku's character goal or defining motto? What guided his decisions to reach his goal? Why did Riku interact with the people he did?
What I am trying to do here is to delve on the fabric of KH, the writing. What themes are present in KH1? What tropes or conflicts drive these characters? Why does Riku play an antagonistic role? If Sora and Kairi aren't the answers then thematically KH fails as a story. The prologue of KH 1 is not just a tutorial for the gameplay but for the characters as well.