This chapter gave some interesting details on Mona. Mona's issue at this point seems to be that she tries to fill her loneliness by getting superficially close to the people around her. By the look of things, she observes people's interests or gets the person to reveal their hobbies to her first, then pretends to be majorly into that same hobby to get in that person's good graces. She did this with both Kurumi and softball and Kokoro with crafts. She also kept her connection to Naoi a secret and came up with an excuse of how she knew Kurumi on the spot. That's what caused the misunderstanding that Kurumi didn't have time to clear up before Naoi showed up. Mona also seems huge into flattery/deception and framing situations in ways to get what she wants. Like lying about her and Kokoro talking about going into the haunted house previously, then pressuring them into having Kokoro come along by saying it'd be better with 3 people, so she wouldn't be scared, when Kurumi later pointed out that Mona wasn't scared at all. It's notable that Kurumi invited Mona to bring her friends along to the festival, but she came alone. If what was shown this chapter is how she usually operates, it's likely she already burnt all of her bridges previously and has no friends.
The chapter cover also alludes to Mona's tendencies. The image looks like it's Mona's different personas that she puts on for each person. It being so cropped out makes its intention less obvious though. The chapter title is also likely about Mona, while also being a reference to chapter 5's title, which in turn was about Naoi. Mona is Vermillion and Void, while Naoi is Red and Vivid. Saying someone is basically empty is definitely a negative thing to say about someone, but Vermillion can have negative connotations too. The color was typically associated with toxicity due to being made up of Cinnabar, which contained Mercury. Also, the cover narration is referring to Mona adjusting her "self" to be what she thinks each person will like the most. This is likely part of why Kurumi sensed something was odd about Mona's emotional distance with her. She was playing a role, not being her true self. It's also questionable how interested she's in Kurumi or Kokoro in particular. She wants to get closer to Kurumi because she knows she's close to Naoi, and she wants to get closer to Kokoro, because she was alone at the festival and needed help.
Mona's plan looks like it's about to backfire though, since Naoi showed up. Because Mona assumed her meetings with Kurumi were being kept secret, she likely assumed that Kurumi invited her because her sister wouldn't be around at the festival. But Kurumi didn't keep her visit a secret, told Naoi that Mona intended for it to be a secret, and told her about Mona's fake/exaggerated hobby. Plus, with Naoi here, Kokoro realizes she was misled about who Mona was, which she clearly wasn't happy about, based on the darkened bubble color at the end of the chapter. Hopefully she doesn't think that Naoi and Mona set this up to pull a prank on her. And it's unclear how Mona will react if she realizes that Kokoro and Naoi don't like each other. If her end goal is getting closer to her sister, then getting closer to Kokoro could start being interpreted as counterproductive.
Kurumi accidentally ended up in the middle here in a lot of ways. She didn't keep Mona's secrets like she asked her to and was basically luring her into meeting Naoi at the festival, because she assumed it was what Mona really wanted. Which might be the case, but Mona was planning to build up to it and to manipulate her sister first, it seemed. And Mona's misdirections to Kokoro made Kokoro think Kurumi was spending the festival with an underclassman who admired her, when in reality it all tied back to Naoi again, which could be a blow to Kokoro's self-esteem. And Naoi probably isn't too happy she was left waiting on the roof for Kurumi to come back, only to find her hanging out with Mona and Kokoro, with Kokoro clinging to her. So, next chapter will likely start off kind of tense.
However, I don't think this is going to be a huge drama. Naoi seemed pretty calm when she showed up. Kurumi already told her that Mona was likely coming, after all. Plus, considering the loud scream SFX and them coming out of the haunted house makes the clinging have a very logical explanation. Also, the festival arc has done a lot to progress Kurumi and Naoi's development. This setup might actually have been for the purpose of getting Naoi ready to talk to Mona and Kokoro. Kurumi told Naoi that Mona was lonely, so Naoi might decide to spend more time to get to know her sister again. And this arc had Naoi rescue Kurumi from Kudou, which Kokoro knows she did. Having hung out with Mona without the bias of knowing she was Naoi's sister, could cause Kokoro to become more open to the possibility that Naoi isn't actually a bad person. So, this arc might end with everyone making peace, like how Kurumi and Isanuma did.
Ultimately, Kurumi's choices this volume could have good or bad results. Either they bring everyone closer, or she accidentally ruined the festival for everyone by inviting Mona. I kind of wonder if this is being done to backslide Kokoro's character development right before Kurumi's mom comes back into the story. Because volume 4 revealed that Kokoro was Kurumi's mom's spy ever since the end of volume 1. And while Kokoro's kept her distance ever since the end of volume 4, it's also been shown that she hasn't gotten over her feelings for Kurumi at all.
I wonder if anything will come of the rooftop key, or the roof likely being left unlocked this chapter. Because that's a very popular spot in media for characters to try to end it all by jumping from the school roof. Or some other significant event might happen there. Kurumi presumably took the key with her, intending to come back, but she never did. And Naoi most likely couldn't lock the door back, since Kurumi still had the key.
Only 2 more chapters left in this volume. Chapter 34 is scheduled for next month if the author remains on schedule. Let's see how the festival arc concludes.
