I've pretty much gushed Riku's feelings about Roxas on the last meme but here's a little more.

Riku's glad to see that Roxas has grown so much. He's always worried that it's left the kid hardened to life but he can still see him smiling and happy, and that's enough for him, frankly. With the recent revelation about hearts and Nobodies, Riku feels like Roxas had even less of a chance at normalcy and kindness and that it just wasn't fair. Frankly, he hates himself because of this revelation, because Roxas should have been allowed to be free no matter what, but ESPECIALLY if he had a heart of his own.

Riku worries that MX will eventually go after Roxas to put another mark on him and to take him as his own, and he refuses to let that happen.