Doesn't Nikadou notice someone in the street before she takes off her bow? Maybe an older family man? If so, I'm guessing she may have had an affair with him and got burned. If not, I think it seems likely that she once dated or at least confessed to the person she loves and got rejected. And she used to wear her hair down, at least with/around that guy.
MC probably caught her crying after she got dumped/rejected (maybe on the phone); notice she was all prettied up and her hair was down. Since then I imagine she's been half-heartedly searching for a more "normal" love among her yearmates. Dating MC is a deviation from that routine, maybe because she's intrigued by someone whose love also isn't "normal". I foresee drama but am optimistic Nikadou will get slowly come to love MC.