When I first read that line about Kase-San saying “that kind of stuff doesn’t suit you, you should stop” I thought it was a red flag/seemed a little possessive (not in a positive way). I think the way page 17 is drawn it’s eroticizing the lipstick application process/it’s something that Kate-San is interested in/something that Kase-San likes... so I wonder if the “stuff” that Kase-San is referring to is the “being mature like everyone else” aspect. I don’t think we really get a sense of why Yamada-San wants to try on the lipstick, other than “everyone at uni wears makeup and looks so mature”. If this is the case, I actually agree with Kate-San. I don’t think Yamada-san needs to force herself to be mature/force herself to be like everyone else. I think she’s definitely got a childlike innocence to her (as seen as her reaction to the plane ride, the trains, talking about sex etc), and to force herself to be more mature because everyone is kind of going against who she really is.
Especially since we’re coming up with the whole past relationships vs new experiences/people, this mature (older me) vs the younger/high school me will be an interesting dynamic to explore. Kane-San and Yamada-San fell in love with each other because of certain qualities/aspects they had in high school. Part of the whole university/post-high school experience is growing up/understanding different parts of yourself/finding out how you want to be. It will be interesting to see if and how they grow to accept the new aspects of each other.