@nbhstcqbvvpmacjrmv
I mean she's a teenager who obviously has some kind of parental issues, it's really not unreasonable to think that she hasn't sorted her feelings out yet. Its a bit much to talk about her being a Bad Person who does Bad Things.
I don't think she was completely correct about slapping Ootaki, but you have to understand that the 80s stuff is, from her perspective, their special connection. For Ootaki to sing a song like that with another girl out of nowhere seems like a direct insult to her. Ootaki wasn't exactly in the wrong at that moment, but he wasn't exactly being incredibly considerate in those chapters.
In terms of the project, she's a shy girl and Ootaki and his friend are both pretty extroverted and know each other well. She also likes Ootaki and is awkward around him. She's also never spoken to him around his friends and Ootaki is rather awkward and opposed about that idea as well. Of course she isn't going to just be instantly comfortable.
The reason for her changing now is not that she is jealous of Ootaki talking to other people or something. Its because Ootaki grew up and acted more maturely instead of constantly worrying about what Yakushimaru thought of him. He's also a bit less concerned about the possibility of his friends finding out about Yakushimaru or his love of the 80s.
Its incredibly obvious that she became more attracted to him because of that. There's loads of panels where it's just Ootaki casually doing something cool and her blushing about it. Ootaki matured, so she had to as well.