Riku had known.
Their group, which used to be Riku, Sora, and Kairi, wouldn't last forever. Not even if he wished for it with his very existence.
It would end up as Sora and Kairi, just Sora and Kairi. He had always known this.
And yet, despite knowing this, he still couldn't help but feel his heart tighten painfully every time they were together. Leaving him out.
He had always known, too.
That his friendship with Roxas wouldn't last forever. That eventually something would happen, and Roxas would slip right through his grasp. Leaving him alone.
And even though he knew this, he still sacrificed himself that day. For Roxas, not Sora.
They were so similar, yet so different. Riku was a Somebody who had submitted to the darkness. He was a Nobody whose non-existence should have faded the day he "died".
But the both of them always watched from afar, watched what was precious to them, even as those precious ones grew more and more distant.
It had hurt, for the both of them. And they had sought comfort, comfort from the pain, in each other. Words weren't even necessary – all they needed to do was loose themselves in the rush of emotion in that one act, and pretend it had never happened after.
And every time, they would end up doing it again.
Even now, as Riku lay in Axel's arms after another "session", he couldn't forget about Sora and Kairi. He knew Axel also couldn't forget about Roxas.
And despite how different they were, they were exactly the same.
