Riku too hoped that Belle would stay- she shared the Prince's love of books, AND she had no interest in trying to marry him, knowing that he was already taken. To top it off, she was smart, and not silly. It made her the perfect female friend for Sora.
"I think it could easily be him," Riku agreed, leaning forward unconsciously and bumping his forehead affectionately against the Prince's. All they needed was... time. Time, now that the war was ending, was over, to get to know each other. Belle wasn't a silly girl- she wasn't going to jump to marry, just because he was kind. She'd want to be in love. And that took time.
"We must make sure they dance together tonight," Riku said, looking to both his Beloved and Tohru. "To give them that time."