I was watching an anime. A boy is in a hurry and he's running, and ends up accidentally hitting a girl. He inmediately apologizes and says 大-大変申し訳ございません! He looks very nervous while speaking. The girls notices this and she looks worried (about him) at first, then she just smiles and says "別に". It's clear that she doesn't want to sound rude at all, she seems to simply want to comfort him and tell him it was nothing and he shouldn't worry. But I often read that 別に is an expression that expresses indifference and a lack of concern, so I wonder when it's supposed to be a perfectly normal reply and when it sounds rude. In this context, is it fine?
It's not wrong to say just 別に in this situation, though it's not particularly polite or gentle, either. It may sound a little blunt or distant as compared to something like 大丈夫ですよ. Maybe the girl was also a little flustered?
別に is a negative polarity item. When said on its own, it can mean "Not at all", "No problem", "Nothing in particular", "Not really" and so on, depending on the context. In this context it clearly means "No problem" or "I was not particularly bothered". It can be rude as a reply for a question like "What's your impression about (something)?"