
So far her goal "seems" to be completely isolate Nana, and be she the only one who stays by her side ...
Anyway, I don´t like Nana, for now, all she does is depend on one person to another...
And maybe Yuyu is a yandere, but she has been his friend throughout her childhood, when she wanted to change assumed that Yuyu would be more her friend, never thought to change and remain friends...
Honestly it seems to me that Nana more than to appreciate to Yuyu like friend, it used to not be alone of unconscious way.

Yuyu is clingy and psycho-possessive. She felt isolated by Nana (cast off) and she did her manipulative thing to insinuate herself and isolate Nana so Nana feels what she felt (ie, teach her a lesson). And now she sees Shioka being a champion/friend to Nana, so she's gonna isolate Nana from Shioka using the power of the group (keep all girls except Yuyu away from Shioka). It's jealousy of the possessive. Yuyu can't bear not calling the shots and keeping Nana at her beck/call/side. She wants to be #1 for Nana and to be the one who leads. Nana wants freedom. Conflict. Yuyu is better at playing the manipulation game (she's very sly and fake), and has been using it on Nana since day one (she admitted that apologizing and saying "I llove you" are magic words, because she gets people to forgive her and do her bidding with those.) Yuyu is toxic, but right now she's acting out of a jealous possessiveness that is just unhealthy. Maybe she'll grow and learn, but she is gonna make Nana's life hell while trying to get back control of their friendship.

Well, l like her more that the boy...
He is so "Do whatever you want, I don´t care anything, ".
The reason he can say such things is that he has never wanted anything difficult to obtain, so he seems so calm. I don´t like those kind of characters that everyone wants them but they don´t want anything ...
What is the purpose of Yuyu with Nana?
If it weren´t because it is a super shoujo I would think the manga would become a yuri twisted or shoujo-ai...
I admit to being rather badly thought out, but since I don´t understand Yuyu, she's the character that interests me the most.