Don't recall if I've said it in here before and I'm too lazy to go look, but it seems to me that Shimamura was probably teased or had some other sort of social meanness done to her, maybe had a good friend turn on her or something, so her reaction was to basically pull back and become a social hermit. I think that was the point of her little backstory about how she was an outgoing and friendly kid; something happened in middle school that made her decide people suck and aren't worth the effort and she did a total 180.
Think that's why she comes over as so disengaged with Adachi, but still has the internal struggle about how to approach Adachi without making their relationship anything more than superficial. Of course she's failing at that, but at a much slower rate than Adachi the Super Gay. More or less, I don't think it's so much that she's apathetic as much as she's trying way too hard to keep Adachi at arm's length--and is failing super hard under the weight of just how adorably dorky and straightforward Adachi is about wanting to be closer.