This has gotten sort of interesting - it's like the tsundere trope, but with extra actual psychology.
Two chapters ago, I was impressed and sort of surprised when An was talking to Shunpei about Lulu and she was so comfortable with him that she was sitting in the seat directly behind him, leaning forward so she was sort of draped over the back of the seat next to him, just comfortably talking and obviously not even thinking about how physically close they were. Then Lulu later asked her how things were going with Shunpei and she got all flustered, and the next time she got on the bus, she threw up a wall between them and went and sat by herself. Then again, with this chapter, her friends started teasing her about Shunpei and she got all flustered again and started out pretty much ignoring him completely, then when she got sort of backed into giving him cookies anyway, she covered it up with the tsundere thing.
I guess it's just sort of strange to see an author relate a chain of events that logically leads to someone acting in a way that could be considered tsundere instead of just sort of arbitrarily flipping a switch on a character and activating tsundere mode. And that's pretty admirable really.