Kairi's letter opened the door to light. It drove out the darkness and brought in a tiny shred of hope, the turning tide that allowed their hearts to meld in a single, breathless moment. As shooting stars, hurtling through darkness, it seemed their bodies, hearts and minds blended. It lasted barely a minute, interrupted rudely by freezing sea water. Then again, by a girl's voice calling. By Kairi's beckon. Our brown haired Protagonist leaped into action, swimming towards his object of affection eagerly, while our Antagonistic secondary Protagonist stayed behind a little, not as eager.
"Sora! Riku!" she waved them up, acting like they had not been lost in darkness, like everything was okay. But it wasn't.
Donald and Goofy didn't let Sora make it up to his knees in the water, tackling him while Mickey assessed the situation and dove for Riku instead, cheering the reclusive boy somewhat. Sora, hugging his friends and grinning like no tomorrow, glanced at Kairi. She smiled down at him.
In a brief instant, Sora became his other, Roxas.
Kairi became Namine.
A soft giggle escaped them, the four hearts, completing each other, but leaving him and another out in the cold. In the shadows of the shaded trees, Riku could almost sense the presence of the disorienting Nobody, Axel. The Nobody sides faded. Wiping some water from his face, Sora glanced up at Kairi again through his hand and grinned wider.
"We-we're back."
Her response was gentle, "You're home." and she reached for him.
Sora had, sometime before or during the short embodiment of the Nobodies, pulled out the one thing that had kept him together, the thing Riku hated above all. Kairi's lucky charm. Looking at it, clenched in his palm, Sora closed his eyes then opened them as his smile near split his face. His swung hand landed in hers, the charm between them.

Sora and Kairi were long gone down the beach, catching up. But Riku was still in the water, motionless, lost in his own thoughts. He glanced off to the side. There, under the palms, was the ghostly form of one red-haired, green-eyed, tan-skinned Nobody who shared a quick, pained exchange with the tormented teen. Holding up a hand, he went to leave, pushing his body off the trunk smoothly.
Watching a moment, Riku called out softly, "Wait."
"Why?" shot back the other. He didn't turn back. "They're happy, leave them be. We're not meant for happiness."
"How can that be?" asked Riku, suddenly angry.
"How? You are a creature of Twilight, neither good, nor bad. No side calls to you but your own... and..." he trailed off then looked at Riku, "his."
Feeling a scowl set, Riku bared his teeth, "And you? You are a being that should not exist. One who does not deserve life!"
"Some life! You think I chose this? If so, you're a fool." hissed Axel.
"You think I wanted to come back? Why did I leave in the first place? Because I thought he had abandoned me for those, those fools! It seemed he didn't care! I was wrong, but it still hurts!"
"At least you can feel that!" roared Axel, silencing Riku. "At least there's a beating heart in that chest of yours, and you can feel love, feel that pain that reminds you you're alive. I envy you. All you who live, truly, in a whole. You have something I will never, and that hurts," he touched where there might have been a heart, "where it shouldn't."
Riku didn't say anything. Instead, his eyes trailed away from the emotional shade, his head following, and he sighed heavily. There was no recall. No escape from the grip Sora would place on him, and that would hurt. A lot. There was no cure, save the happiness within those sky blue eyes for which that beautiful boy was named. Even that, was not much or for long.
He and Axel shared that in common. Both trapped in cages shaped of the same figure, the same body, different mind, different reasoning, same person. Two sides. A flip-coin that, once whole, was not capable of being separated.