Maybe, once upon a time, Riku thought of Sora as some kind of brother... but experiences with his own told him that no, Sora wasn't a brother. He was something very different than that. Until now, Best Friend, was the only word he could come up with for it.
"Is that a hint?" Riku teased a little, but then sobered. "I like kissing you, and being kissed by you." It hadn't been as weird as he'd feared. "I think, maybe, the smartest thing Tidus said was to not force anything. To just let whatever this is happen." He was worried that he'd never want Sora, physically, like he might want a girl, and that was scary. Because Sora was so important to him. But, like Tidus said, there was no reason to rush anything.
"The only person we're responsible to in this relationship is each other." They didn't have to impress or make anyone else happy.