Disclaimer: I don't own Kingdom Hearts.
He would lie for it. He would steal for it. He would fight for it. He would drag himself through the seven levels of hell for it. He would stain his hands forever red, break his body to pieces, keep chasing after it no matter how utterly it destroyed him in the end. And if worst came to worst, he would die for it. Happily.
This is what Sora would do for love, and he realizes that this is also what he would do for Riku. And only for Riku.
It takes Kairi years to fall out of love with Sora. It takes years for her to swim out of the river of denial she'd drowned herself in and to realize that, in essence, he is no longer the person she loved.
It takes even longer for her to realize that Riku and Sora are in love with each other. They're a twisted version of the storybook soul-mates she once believed herself and Sora to be.
Kairi wonders what will happen if Sora ever manages to bring Riku home. She knows they can't live without each other. But she doubts they'll be able to live with each other either. Not any more.
They stand now at the point of no return, but it's not like they haven't been here before. Riku doesn't want to kill Sora anymore than he did in the beginning, all those years ago. He doesn't even want to fight Sora. Whether he can kill Sora or Sora can kill him is not the point. They've been killing each other for years, from the inside out.
The sight of Sora makes what's left of his heart ache. Sora isn't supposed to hurt people. That's as good as Riku's job now, after all he's done. Sora is the one who's supposed to protect people, to save them.
But after all this time, Riku isn't still sure how he's supposed to say: "I'm the one, Sora. I'm the one who needs saving now."