Let me guide you through the actual dialogue of that one, single interaction we saw.
Naminè: So we meet again.
Riku: You made me a promise.
Naminè: I remember. It was to look after Sora. I'm sorry...I'm not sure I've kept that promise very well.
Riku: Can you tell me what's happening?
And then she goes to explain the situation with Roxas and Xion, because Riku and her got straight to business right away. I mean, hell, he doesn't even bother with casual conversation with the girl. Does that dialogue sound like people who have been seeing each other for days? Have kept in touch? Had been spending time together? If they had, then they wouldn't be having this meeting, because Riku would have been aware earlier. And I'm pretty certain this was on Day 225-250 something. That's more than six months and near to a year. Honestly, they barely sound like friends.
This isn't just something vague and that "we don't know about". I'm not pulling this out of a hat. It's literally within their conversation that they have not seen one another, hence, the "we meet again" and other points of note that show they got reacquainted.
And I don't see what's not to understand. Why would Nomura need to protect or refrain himself about the relationship of two characters who he sees as not romantic in nature? Granted, it was his dumb fault in the first place by portraying it the way he did and expecting that somehow the shoujo manga-esque scene had something to do with Riku Replica. But that was his intention, for it to be about Riku Replica...somehow. Some (not all, because not everyone had shipping goggles) understandably questioned it, and so he went out of his way to make things clear. Because Riku Replica matters now and so he matters to Naminè now after a bunch of games of not giving a shit about him. Hence, why I call it a horrible execution. However, it was his horrible execution and he wanted everyone to know it was about Riku Replica, not Riku.
Though I found it hilarious that he went as far as changing their expressions. Mans was serious about Repliku. Just not about female characters I guess
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You can say that about any character. Which is why we even had the Riku/Aqua discussion and why this thread exists.
What's your case here?
To each their own. But there's literally been nothing romantic between them. There hasn't been for 8+ games, he spoke to her like they were coworkers, he manhandled her while she tried to speak to Roxas, he allowed Diz to treat her the way he did, and only stepped up against him when he was ordered to kill her. He didn't mention or remember her in KHIII (unlike Sora, Kairi, and even Roxas), and she wasn't a priority to him until Riku Replica made her one.
[sad noises]
Sucks you have to be in a trio to matter to people.