Gonna try to condense all my rambling thoughts about Kokoro since it seems like we are reaching the apex of her character arc (it'd be pretty hard for her to come back at this point but I suppose it's not impossible lol maybe Akane can fix her) please ignore the incoming wall of text if you so choose but it will all be Kokorover soon-
The chapter cover for chapter 17 is extremely on the nose, but it basically sums up Kokoro in a nutshell. Fundamentally Kokoro seems to view Kurumi as as a thing, consciously or unconsciously, like she's a doll with a string on the back that says a set series of phrases when you tug on it. When her doll doesn't say the thing she expects, she freaks out and starts wondering who or what did something to her doll, running circles around in her head wondering if it was something she did, or something Naoi did, before obsessively settling on Naoi being the culprit when all Naoi really did was pour gas on a fire that was already starting to burn. She was incapable from the start of thinking that it might be something wrong with Kurumi, which is another way she doesn’t recognize Kurumi’s own agency and ability to choose her own path. Dolls don’t just break on their own of course.
Kokoro was bullied when she was younger which ofc is a lasting trauma (not that it automatically turns you into yandere, helps explain but doesn't excuse lol). Kurumi, being the good kid that she was, quickly became Kokoro's object of adoration. In a way she starts to prop Kurumi up like she's a super hero. You could even bring that back to the doll metaphor, because super hero action figures are dolls too after all. She cuts her hair specifically to get Kurumi's attention, not because she wants to open herself up to everyone. She does similar things later, changing her 'style' to try and get Kurumi to notice. Even rabbits, which is really the one thing we saw Kokoro have an interest in before meeting Kurumi, end up being something she associates with Kurumi. I kind of think this is where Kokoro could have changed into a better person. Akane points out that Kokoro has always been the crafty type and the carrot hairpin is obviously an homage to her love of rabbits. Somewhere in there is just a shy kid who likes making rabbit hairpins, but she’s spent her life picking the wrong options so many times over in little ways that she instead built her personality around her obsession with Kurumi. Oddly enough, if Kurumi hadn’t thrown away her phone and she and Kokoro had a talk sooner after what happened, things might’ve turned out differently. She didn’t really know how to respond to Kurumi after she slapped Isanuma (based and well deserved) so there was a crack that maybe could have been used to break her ridiculous image of Kurumi. Instead Kokoro learns all the wrong lessons from that lesson and ends up having way too much time thinking about what she could have done and twists herself into a yandere pretzel.
When Kurumi started drifting apart from her classmates, Kokoro chose to also basically isolate herself and only spend time with Kurumi, who already barely had any time for herself. Probably safe to say she didn’t have much if any free time on her days off that wasn’t devoted to studying. This could point to a few things- one, that she never really considered her friendships with other people to be all that important, two, she's so consumed with Kurumi that she's willing to throw away her relationships with everyone else. Having other friends was always incidental to being around Kurumi first and foremost. With how busy Kurumi is shown to be, and how surprised Kokoro is when she asks her if she wants to hang out somewhere for a bit in the first volume, I think it's not unreasonable assume that Kokoro spent most of her time alone after they drifted away from the others. She's not shown to be in any clubs and distanced herself from the other kids, meaning her social interactions were probably limited to walking to school with Kurumi and whatever conversations she had with people during the school day. I think since Kurumi was also functionally alone it was easy for her to imagine that the two of them were already in some sort of ‘special relationship’ and that she wasn't actually alone. Everyone else left Kurumi behind but not Kokoro, who seems to think this all makes her worthy of special attention. Not seeing Kurumi a lot and no one else also gave her plenty of time to fantasize about her perfect doll version of Kurumi without a healthy does of reality getting in the way.
Even though Kurumi is the one who is crushing on Naoi, Kokoro is borderline obsessed with her (not in a romantic way ofc). She blames Naoi for everything she perceves as a change in Kurumi's behavior, and she really starts to lose it when Naoi teases her, implying that she and Kurumi may have had sex or are in a relationship. She reads Naoi's short letter to Kurumi and then withholds it from her which is a violation of her privacy and agency. She even tells Naoi straight to her face that she didn’t deliver the letter when she could have just pretended she did, but it was more important to her to make sure Naoi knew Kurumi was her ‘girlfriend’ now. Kurumi asking if something happened to Naoi is the thing that causes Kokoro to confess to Kurumi because she can't control her jealousy. Naoi gets in the way of the special, almost exclusive relationship she’s used to having with Kurumi. Naoi in her eyes, is trying to steal and break her doll. Even when she very tepidly stands up to Isanuma at the end of the second volume, Kokoro still brings everything back to Naoi, implying that the rumor about Kurumi is somehow Naoi's fault. At the beginning of this volume we have her 'inspecting' Kurumi's body, obsessively looking for the parts of her doll that have been 'tainted' by Naoi. Akane pointing out that Kokoro might be viewing Kurumi in too positive of a light is met with immediate, though somewhat tepid backlash. Kokoro blames herself for asking Kurumi out, which eliminates Kurumi’s agency from the situation again, even though Akane correctly points out that Kurumi shouldn’t have agreed to date her just to avoid hurting her feelings (not that Akane knew what kind of pressure Kurumi was under at the time). Kurumi wasn’t able to make a rational decision at the time because of her mental state, and hilariously when she does try to correct and apologize for her mistake Kokoro refuses to listen to her and says Kurumi clearly isn’t in her right mind and can’t make her own decisions, but Kokoro has basically been stopping her from making her own decisions since the first volume, telling her mom about the video despite Kurumi pleading for her not to. Kokoro ends up blaming this on Naoi as well.
Kuwabara pulled it off pretty well overall I think! At first Kokoro can very easily be viewed as a well intentioned, if oblivious idiot, but by the end of volume one the tone is definitively set when Kokoro betrays Kurumi by going to her mother, that Kokoro is not Kurumi's friend or ally, and is instead going to be an antagonist. It’s the perfect flip from Naoi antagonizing Kurumi to Kokoro. Kokoro can’t actually be a true friend to Kurumi because she doesn’t see her as her own person, with her own strengths and flaws, whereas Naoi sees Kurumi’s flaws right off the bat and eventually accepts all of her.
I think my favorite part this chapter was the thing that really started making Kokoro crack was Naoi very reasonably going "Just let Kurumi tell me what happened", because on some level Kokoro knows what she's doing is fucking unhinged, and Kurumi voicing what Kokoro has done outloud to Naoi would make it real. She doesn’t want to hear what Kurumi really thinks. She doesn’t want Kurumi to have the agency to do bad things or like bad people because that would mean the perfect person she’s built her personality and life around was never perfect in the first place. If she can blame herself or Naoi, she can still rationalize Kurumi being corrupted by some outside force, rather than Kurumi just being a person and sometimes making bad decisions. Kokoro’s so deep in the sunk cost fallacy that she strangles Naoi instead of letting Kurumi break her glass cage.
That was barely coherent but I needed to get it out of my brain. Anyways! I think another possibility is that Kokoro gets snapped out of her murderous episode after noticing that Kurumi had fainted and actually runs away in a panic once she realizes she tried to kill or are least seriously injure Naoi. I don’t think that’s particularly likely but who knows. It’d definitely be more satisfying for Naoi to take her down a peg or two though lmao so I’m rooting for that