I'd already been spoiled for Naoto, I'm afraid (though not by the artbook, as so many other people were). Still, I probably would have guessed within the first appearance, just from the voice acting.

But yeah...I mean, I can generally handle a certain amount of moe, but she's a fricking HIGH SCHOOL STUDENT who looks like she's about NANAKO'S age (which makes dating her SOOOOO much creepier, when you think about it).