I have a feeling Mayuko was sexually assaulted when she was in elementary school, reason for her acting like that and Ayahiko rape made her relapse that moment... damn.
Takagi and Mayuko moments were actually pretty pleasing, even made kuudere smile for a brief moment. She may actually help her more than Makoto, who managed to blend himself with Ayahiko personality.
I'm surprised the password was Makoto birthday, didn't he hate him? Although if he kept secret from Mayuko, she wouldn't figure out that's his password because he hated his brother.