Good girls always remember to pack an umbrella, or sometimes even two, so that they and their forgetful kouhai won't catch cold from walking home in the rain. They never walk home bareheaded, faces turned up to laugh at the rain.
Good girls never forget their bentos, either. Good girls know that a lunch from the vending machine is no kind of lunch at all for a growing boy, and they are never reluctant to share with anyone who has a fancy to lean over a shoulder to pick out a bite, or two, or ten.
Good girls join the school disciplinary committee, and make it a top priority to curb the delinquencies of their classmates--even the ones who have no discernable regard for the rules, for authority, or, quite possibly, sanity. Good girls don't become the managers of sports teams and turn a blind eye to the excesses of their teammates as long as no one gets (seriously) hurt.
Good girls never swear, and they don't put up with boys who do. They certainly don't make note of the more creative curses they hear, and try them out later, just to taste the words spicy on their tongues.
Good girls think smoking is a disgusting habit. They don't admire the way a cigarette looks when it's held between long, careless fingers, or the way a lighter can cast sharp-edged shadows over angled cheekbones.
Good girls are careful with everything: their friends, their kouhai, their responsibilities, their reputations, and their hearts--especially their hearts. They wouldn't dream of letting themselves fall in love with the devil himself.
Maybe, Mamori decides, sliding her fingers into Hiruma-kun's hair and hauling him down for another kiss, she's had enough of being a good girl.