Responses
I'm pretty sure that's because Nao wanted to inherit whatever her name is (fml)'s last name. That way he can inherit her family fortune/business, because if she were to take on his last name he would make no profit from it.
Remember, he originally used her for something like that or whatever. It's hard to explain
why is it that its nao's surname thats changed? this confused me, do males in japan "inheret" their wives surname or is this just a mistranslation?
http://iweb9.mangapicgallery.com/r/newpiclink/miseinen_dakedo_kodomo_ja_nai/7/2f9f5d6c26021be0d95dc31d7d1576a8.jpeg