My personal theory about Nanase and the future of the story. Warning, heavy spoilers ahead!
Nanase is still a mystery to this day! I guess her full background will be revealed in the last volumes of year 3, near Kiyo's graduation. Also remember that Kiyo asked Amasawa to investigate her, and she should be competent enough to produce reliable results.
Even Kiyo thinks that her whole story of Matsuo is BS, and even if it were true, is not important considering the future.
I'm 90% sure she's just a Tsukishiro spy, this means she will join his side outside the school, and what side is it?
Here's the beginning of my theory.
previous groundwork. In the year 2 history, Koji realized that Tsukishiro was not really interested in expelling him, he recognized he had enough skills to expel him if he really wanted it....but he didn't, so what was his real goal in school? maybe he just wanted to measure him. for what purpose? well, I think he's preparing to betray Ayanopapa.
He will most likely switch sides to Koji once he's out. I based this in the last interaction he had with him, in the ship. Also it looks like Koji does trust him. This is also my theory, but remember the last interaction Koji had with Nagumo?
In the end Koji recognized Nagumo as trustworthy, and he gave him "a message for someone outside the school" he also said "It's not for Manabu". Who else can Koji contact outside the school that's also known to Nagumo?
Exactly! Tsukishiro! who else could it be if not him? There's none else!
This means that Koji will likely form his own faction outside the school to oppose his father!
The members of the Koji faction will likely be: Tsukishiro, Arisu, Chairman Sakayanagui, and....only if they get involved in politics, Manabu, Nagumo and maybe Kiryuin.
And since Nanase is loyal to Tsukishiro, she will naturally join Koji's faction. The only problem I have is ....she still has one year remaining in the school, she can just drop out but....that could be a problem because there's none competent enough to take the seat as the next student council president. If Koji cares about the school enough to worry about it, he can just tell Tsukishiro to order Nanase to remain in school and become president. But that's still up in the air.
Also remember that she still wants to expel him, I guess she will make a last atempt later but will fail, and then everything about her will be revealed.
There's still potential problems in my theory if we consider Ishigami and Shiraishi but I guess that will have to wait for the moment.
What do you think about my theory? delusional or possible?