Once upon a time, there lived a girl named Zheyne who didn't own Kingdom Hearts. The end.
Sora doesn't see the issue, but Kairi does. He can't live in their world, not after what he's done, even if Sora doesn't understand why, even if Kairi says he's being silly. They don't mean to exclude him.
They don't mean it.
But that doesn't mean it doesn't hurt.
He watches them glance at each other, snag pinkies when they think no one is watching. He sees their secret smiles, the way Kairi's face lights up when Sora is mentioned. He knows that Sora has been in love with her for the last five years, and has loved her for longer than that.
He pretends he isn't bothered. He pretends he is the strong one again. He pretends he isn't jealous.
But he isn't strong any more. And he is jealous.
But Sora doesn't need to know that. Sora shouldn't be expected to shoulder his burden, not after what he's done. Not after Sora's been expected to shoulder the weight of the worlds, to decide the fate of the worlds.
Kairi's the pure one, anyway. He deserves her, and Riku doesn't deserve him. Not after what he's done.
Their lips meet, and his stomach rolls. Their rings match, and his stomach sinks.
Their vows are sung, and his heart dies.
Sora doesn't know. Sora doesn't know that during the night, Riku dreams. When he's alone, Riku fantasizes. When they're together, Riku tries to forget, because love is not selfish and he's in love.
"I now pronounce you man and wife."
In retrospect, he should have known it would turn out this way. Especially after what he's done.