She is NOT taking this well!
When I look at the bully Trio, you can virtually draw a spectrum between the three of them. Based on their level of maturity
Tsubami being the immature
Kanako being the mature
Chisaki was always, an anomaly, she wasn't nearly as refined and elegant as Kanako, but she was nowhere near immature to the level of Tsubami
Whats more is: Most things in the story that defined her as either mature or immature was purely coincidental
She's the ringleader which gives her an air of maturity. But she has a moe fang, which is a sign of immaturity (moe cutey stuff y'know?)
She's the main initiator of the bullying, which makes her immature, but she's sexually active which, sort of? gives her an air of maturity
She's strong, clearly capable of handling herself in a fight, even able to take on men. But she's clearly very vulnerable emotionally. Unable to develop a meaningful connection with anyone
although in all fairness I've never seen anybody able to develop a real connection with another purely by having sex with them... But you know!
Personally I feel this makes her the most interesting character in the cast.
She's an outlier by chaos itself. Which makes her, dynamic to a fault, but that fault is what manifests an actual plot device and makes the series so interesting to engage with in the first place.
This backstory, STRONGLY reinforces this!
Chisaki doesn't bully Ueno out of spite or out of sport. She does it because it establishes a "clearly very" rocky foundation of Ueno and Chisaki's romance.
But what drastically differs between the two of them, is perspective.
Perception is everything!
And as a result, it has created a loud noise of activity, the two of them being the epicentre of this massive chaotic bubble that is about to burst!
Kanako, having intimate feelings for Chisaki, has been driven into a despaired state, seemingly almost Yandere, as her love and feelings is not returned. She is mature and smart enough to obviously figure out it purely rests on Ueno, but since he is clearly too passive to show the initiative necessary to start or stop this all (speaking from her perspective in case that wasn't obvious).
As a result she is dragged along for the ride
Tsubami, immature as ever, has been driven into a corner where everything in front of her is undiscovered territory. Her curiosity getting the better of her, it sends her into a spiral of newfound... kinks that she never knew possible. Being a sheltered child, she clearly sees this as a gap, a rift separating her from her friends. To her this noise, this activity is like a gap that keeps widening and the only way to close that gap, is to move forward.
To some extent, the story hinted it, but it was always essentially a matter of who was going to be the one to cause everything to spiral out of control. And for a short moment, it didn't, no matter how scandalous it got!
At a quick glance, it would appear to be that it was because Ueno was just idly tolerating the machinations of every character's whims and... well, sexual exploits
But something stands out here, which I really hope the author delivers well.
And that's Chisaki
Now I'm not trying to dictate a love affair to this series or anything, but I feel that this dynamic is somewhat under-rated.
This story, like it or not, isn't PURELY about Ueno. It HAS to be about her as well!
I get people like the main character a lot. I get people like when the male character is the core of a story.
But Ueno's story, doesn't function WITHOUT Chisaki!
Yes, Tsubami and Ueno looked like they had developing... exploration things going on. But why? Because Chisaki!
Yes, Kanako nearly went full Karen Super Saiyan and locked Ueno in the 'basement' (metaphorically), but why? Because Chisaki!
For Chisaki, the stakes are high. Very high!
Unlike Kanako and Tsubami... she's bound by the same rules and limitations as Ueno, for better or worse! And I think this is a glossed over part of this series, but Ueno is entering a point in his life, than Chisaki has already gone through!
You have this strong female character, who's a polar opposite of Ueno.
Ueno is portrayed as reserved, shy, meek (i don't think he's weak but) and inhibited
Chisaki is portrayed as strong, independent, confident and extrovert
They are completely different but they are bound by the same rules.
If the story phases her out, the story loses its grip!
And as much as Ueno and Manaka seem to get on well and huge developments there.
I am team Chisaki for this one
She is every bit of a main character as Ueno.