I felt the same way when I first read it. I will was like wtf is wrong with this girl? As for Setsuko, we need to remember this story is from Yukinos POV and her initial impression of Setsuko is the cool,mature lady. As the story progresses we see them opening up to each other and their real characteristics start to show. Which is why I agree she was such a realistic character in the fact that she wasn't perfect and had her own worries, jealousies, and insecurities.
I was on the same page a you Razzl for my first few times reading this so I get get you! :)