I think there is no straight explanation for how Riku feels about Sora. He is deeply in love with him. I also believe that Sora is aroace (hc) and would not enjoy dating anybody. Friendship is his power!
That being said, after all the drama of Riku settling in with his relationship with Sora instead of being weird and angsty about it, they seem to be completely content in the relationship they have. I don’t get a sense of longing from either of them in the later games. They care deeply about each other but they don’t need more than they have. I like to think that Riku is asexual and, while being deeply in love with Sora, doesn’t need more than the really close friendship they have. He knows Sora can’t provide him with anything more than that and he respects that. He knows Sora does love him a crazy amount and it’s enough for him, even without explicit romance.
