Wow, I did wonder why the age difference was so huge and why he was acting like a typical(?) "lovey dovey oniisan" with his kid neighbor. The Japanese definitely have weird notions about what constitutes appropriate behavior with respect to minor/adult interactions (even the sibling relationships are a bit odd). But perhaps it's because I'm a Westerner that it's strange to me? Idk. I watched a program on Amazon (with James May) where the Japanese tour guide said that the Japanese "are a nation of perverts". Idk...
Yuu is attracted to her now, still... but he also said that she's still that "ridiculously pure" kid at her core. Is that what he likes about her? I'd say that his attraction to her now is because it's Marika, and he doesn't care if she's an adult, but it's abnormal to even think romantically about a 12 y.o., and Marika looked mighty young laying across his lap! Hell no. And even if he was a minor, it still would be pedophilia... the DSM says the afflicted has to be at least 16 and the victim/ object of attraction is 5 years younger. So he fits it. How intuitive of Marika to suspect he was a lolicon!
But perhaps he became attracted to her because he was lonely as an only child so she was someone he doted on, and it got twisted up into romantic feelings? I'm sure, regardless of the provenance of his attraction, he's gonna be normal in the end after all.