@burningknuckle
I don't get it too much myself but when the newspaper club president confronted them Aozora admitted she thought the missing page from the Banana Files were with the playgirls. From there Kasumi jumped to the conclusion that Aozora got close to her just because she suspected Kasumi specifically of having the missing page. Kasumi threw a tantrum after that and a rift formed between the two. She feels bad because she realized she overreacted and that all she wanted was for Aozora to be honest to her.
TL;DR: Some very convoluted shit and Kasumi being stupid.
@Edgerunner
I may have missed some but all chapters relevant to Aozora seem to be
17, 22, 23 (pre-kiss)
34, 35, 37, 49, 54, 67, 68, 72.
Aozora is also mentioned in ch 69.