Rikuo has seen a lot in his life - a lot of weird shit, whether normal-weird or supernatural-weird. He's seen a lot, so he's always been pretty hesitant about saying that he doesn't believe in things. He's seen a lot, but Kazahaya pretty much takes the cake as far as disbelief goes, and has since the day they met.
It defied belief that Himura Rikuo would deign to pick up some kid he found in the street; it defied belief that said kid would be offered a room and a job like he was, but that was Kakei-san for you. Totally unpredictable, presumably because he could predict just about everything else. These things tended to balance. So maybe that wasn't Kazahaya so much as life, but there are other things that make the kid completely unbelievable.
Like his total lack of knowledge about the outside world. Rikuo had never met anyone who didn't know how to cook rice, who was so completely out of his depth in a classroom - who wasn't even able to respond properly to playful sparring in a conversation. Everything Kazahaya did had to be explained to him first, like he was some kind of retard - but the kid was actually pretty smart. Rikuo knew he was smart. A little slow, maybe, and definitely an airhead, but he was intelligent enough. Which is maybe why the most obvious thing about him is the most ridiculous, the most unbelievable of all.
The guy is completely oblivious.
Rikuo is not kidding. Completely. As in, there are things you'd have to wave neon signs in his face to get him to notice, and even then, he probably wouldn't pick them up until you hit him with them. The really annoying thing, there, is that the things Kazahaya totally ignores tend to be, really, pretty damn important. Or at least, pretty important to his health and safety - pretty damn important to Rikuo, in any case. And it drives him nuts, because he can't even do anything about it - he can't change the way Kazahaya looks at the world. It's not something he broke that can be glued back together - this is the way the guy came, and there's really no glue in the world that can change perception like that.
Doesn't keep him from trying, though. Doesn't keep him from extending his hand at the right moment, from pointing things out ahead of time to make the kid feel like an idiot. Hell, maybe if he gets embarrassed enough, all the blood that rushes to his face might kick-start his skills of observation.
It's worth a shot, anyway, Rikuo reasons. And even if it doesn't work quite the way it's supposed to - Kazahaya's pretty cute when he's flushed and angry like that. So it's not a total loss.