I don't think it's actually been confirmed, but there has been a lot of speculation in the forum here that Akiho (the black haired girl) is his blood related sister from the dad's first marriage, which was split up when they were too young to remember. It's possible that neither of them are aware of that at this point. Also possible that Akiho is 100% aware and hiding it. MC has no clue though. The main clue is that they look a lot alike.