Riku sits on his little island, and watches Sora. I don't think Sora notices, but I do.
It's hard to spot, because most of the time, the three of them are together. Sora is so happy to have Riku and Kairi back, he'd play with them all day if he could. But neither of them can match his energy, so sometimes they go off and do something more restful than Sora's almost frantic schedule of running-races-catching-fish-building-rafts-sword-practice. Kairi spends her breaks looking for seashells, or flowers. Riku spends his watching Sora.
I think Riku's the type who obsesses about things. I saw it in his eyes the first time we met. Being in Organization XIII taught me to recognize obsession when I see it. (Actually, being in the Organization taught me to recognize just about every kind of crazy there is; it was like living in a psychology textbook.) I think the thing stuck in Riku's head now, that he thinks about over and over and can't stop, is Sora's heart.
I can't blame him, really. I never imagined it would be like this. Of course Riku wants to touch Sora's incredible joy. And for him, the strength would be almost as good, the way Sora can forgive anything in an instant, because he knows nothing can really hurt him. I think Riku needs forgiveness like that. I think it heals something in him that's been broken.
Riku tries to keep his funny obsession with Sora a secret. I don't know why. It's not like Sora would be angry - Sora's never angry. But it's only from a distance that he watches Sora. He waits until Kairi's somewhere else and Sora's asleep to touch Sora's hair.
The funny part is, Sora's faking it. He likes the comfort and closeness of Riku's fingers brushing softly through his hair. The first time, he said something about it, and Riku told him he'd been dreaming. So now Sora knows Riku won't do it unless he pretends he's asleep. He usually does fall asleep eventually, warm and safe, feeling Riku's hand and Riku's eyes.
Maybe Kairi knows, too. Maybe she's not always as far away as Riku thinks, when he does that. I don't know, I can't see when Sora's eyes are closed. But I think it would be hard for these three to keep secrets from each other.
They let Riku think it's a secret, though. They let him believe he can hide an obsession with the light the way he did his obsession with the dark. It's obvious to me, it shines through like a sun in a paper bag. But they pretend they can't see it, let him keep a few secrets, give him time to heal.