...there's just something about it that rubs me the wrong way? On paper, this sounds like it should be really good. It's cute wholesome single-heroine romance. The MC is appropriately rational and doesn't have any terrible traits that I dislike, and the heroine is cute and fun in good amounts.
...So why do i not like this? I'm honestly not sure myself. It's weird? I think it's a combination of things. One, both leads are stupidly rich and part of a high-class family, which means its impossible to empathize. The MC is a loner because he's used to people trying to get to know him because of his family, so he puts up a pretty harsh wall (though he's never cruel, he is overly harsh and intentionally attributes the worst to others). Enter the whirlwind that is the heroine. Per the title, he thought she was a boy all this time, and was literally his only friend his entire life. Well, now she's returned, having literally spent the last 10 years training to be the perfect wife for him, despite knowing that he has no clue of her gender. She comes off to me as extremely two-faced, and I really can't stand it.
I don't know WHY though, because she's basically a tsundere. She's hopelessly crushing on him ("In love" with someone she's never met for 10 years, but that isn't even TOUCHED UPON), but plays it cool all the while moving in with him and doing anything she can to infringe upon his space. I think that's the reason to be honest: he comes off as a 'fake-loner' who is socially adept and does it because reasons, but doesn't actually want for friends, and she comes in and just intrudes upon his life in such a way that if he were an actual loner he'd have a breakdown.
I dunno. First world problems I guess. Many aspects of the story I should adore, so i can't help but magnify the issues I do have with it? Something like that?
Anyway, not sure, but i can't rate this as a 10/10. YMMV. Take my review with a grain of salt, see if you like it yourself.
Enjoy!