The Good Girl

Mio Yukimura, 17 years old, is a normal high school girl. She has normal working parents and becomes an elder sister in her senior year. She has good marks and is admired by her teachers and schoolmates alike. But there is something about her that makes some people curious and that is her obsession about being a "good girl". Why does it make someone wonder if it's normal to try to be a 'good girl'? If it isn't, then why is it so important for her to be one? To what extent can she go to keep being one?