Kahara is like a younger version of Hinohara. Because she's younger and also still a student, she's not bound by any societal pressure, or even legal reasons, to keep herself in check, like Hinohara is. At the end of the day, despite the theme of this series, Hinohara still is an excellent teacher, who more or less saved Nagai from the useless path of a delinquency (though he never was that bad, objectively speaking, but he could have easily become worse in high school) and managed to foster his unique skills and make him believe in himself. It's precisely because she's an excellent teacher that she finds it nigh impossible to step too close to Nagai. Kahara, obviously, doesn't have such inhibitions. In some ways she's not only like Hinohara but she's also like the female version of Nagai himself, rough around the edges behavior wise, with certain delinquent traits. That's not a bad sort of personality for an artist, anyway.