@BOBBYERTLY The minimum age thing is probably relevant for almost every age Nei could be. The thing I'm pretty sure of is that their absolute minimum in this regard is 13, it's the old age of consent that's still on the books and other laws have been added on top of since (mostly to make the effective age of consent actually about 16-18, that gets a bit complicated so let's not dive into that here).
I took a look back towards the beginning of this series and Souta appears to be in 5th grade, so about 10-11 years old. Her grade isn't mentioned yet as far in as I was looking, but if she's still a first year middle schooler who hasn't had her birthday yet, she'd still be 12 and neither of them are the minimum age yet so it's arguably safe in that case. But if she's anywhere from 13-18 herself, it's still basically the answer I had given that other commenter: yes, it's possible there's a legal issue here despite them both being minors, but that is depending on the more specific Japanese laws that I'm not completely familiar with.
She's perhaps best described as Schrodinger's criminal without more information on her and the Japanese laws, until her age is observed we aren't sure if she's guilty or innocent in the eyes of a law we may or may not be able to find enough details about. Of course, even if she is technically breaking the law nothing happens about it unless somebody reports it one way or another.