Honestly, words of truth. I had a best friend like Yuyu who did the same things similarly to me (not the copying trend though). Except, unlike Nana, I had half a brain and eventually got free of that friend’s manipulation and bs. When I ended the friendship, she cut her losses in a way by blocking me on everything instead of the other way around. I didn’t get the satisfaction of blocking her which I think she knew that I wanted, seeing as how I blocked her on one already prior to ending the friendship. And since she is heavily emotionally abusive and doesn’t blame herself for anything, she’s likely already targeting someone else. It’s been almost a year but the scars are still healing from what she caused. But the good thing that I’m grateful for everyday is that she’s gone now.
The ending is an obvious letdown but seemed like everyone knew that since the last three chapters. I don't think Yuyu is the type to change. She didn't get what she wanted from Nana so she fed her some type of emotional pull one final time before vanishing again and even though she is cutting her loss I can't see her changing herself for the better. She will find some other girl to manipulate and control elsewhere.
Hopefully when Yuyu does return Nana will be a much stronger person and not try to have much to do with her.