Thanks for the translation.
Nazuna was introduced and killed within one volume, and it seems just a chapter later the characters are already starting to get over him.
Then we quickly learn that she felt literally nothing for Nazuna even after sleeping with him, then he dies instantly after with her randomly acting like he did mean something to her (but he really didn't, it was because of mc) and now a chapter later they are over him and she is back to square one feeling all alone.
Not really. There was a time-skip to the next season, and they're
still thinking about how Shuuto had Mimosa kill Nazuna as if it happened yesterday. It makes sense that they'd be getting over him after the time specified, but they
aren't. Still, that's also fine-- what was off to me was that every discussion point in this chapter felt like stuff that one would expect to be brought up the day after, if not immediately after the ride away from the murder scene. If the time skip wasn't just a waste, it feels like it was meant to shave off time from the promised 75 days this story is promised to span without having to deal with the burden of coming up with new sequences and perhaps character inflection points.
Nazuna also clearly meant
something to Mimosa, and aside from that, the majority of what Mimosa is feeling over her killing Nazuna is that the event had Shuuto show a side of himself that was very unlike her original impression of him that inspired "warmth" in her.
Some stuff that's better as a general comment that I happened to make in response to yours:
Both characters are chasing after "warmth" through the relationship they have with each other, but more than that, it appears they're actually finding that "warmth" in idealized versions of each other. When Mimosa talked about her past with Shuuto in chapter 3, he hated the conversation because-- essentially-- he couldn't reconcile that new information with the understanding he already had of her without also losing familiarity with her. His machinations to get Nazuna and Ikari killed were partly his possessiveness but also the fruit of him trying to prevent himself from no longer recognizing Mimosa. He tried to accomplish this by becoming something he figured was "similar" to her, meaning he sought to become a killer like her. Also recall his sexual thoughts towards Mimosa-- when he recognizes that he isn't disgusted with Mimosa's naked body, his rationalization involves
worship (ch. 4, p. 30). He's completely unable to reconcile his desire for "family" with Mimosa with the fact that he's irrationally possessive of her sexuality (ch. 7, p. 23) and is just unable to admit to himself that he's sexually attracted to her
or that it's a factor in his motivations.
The irony is that Mimosa has a similar issue-- the Shuuto in which she found "warmth" was the one that was way too happy eating cup ramen with her, not the one barking at her to kill an old associate as part of a ritual that only made sense in his head. We know Shuuto always had a personality that could lead to him doing something like that, and even Nazuna could pick up on
something wrong with him. However, Mimosa never really reckoned with trying to understand him seriously, and now that she's been forced to see and confront these traits of Shuuto's, it's compromised both her understanding of
and familiarity with him.
Neither of them is comfortable with this-- both of them want to feel that "warmth" with each other, which is why they still haven't separated after all this time and why Mimosa is suggesting they do something that they agreed to in chapter 3-- and were working on from that point-- but probably gave up on during the time skip. It's also why Shuuto was capable of recognizing that he probably screwed up and very much resembles his father at this point.
But they're still not trying to take a good look at each other.
Nazuna's purpose, in hindsight, is to telegraph to the reader what exactly needs to be paid attention to: Mimosa and Shuuto aren't quite looking eye-to-eye, as he says. Shuuto's an unsettling existence, as he says. Shuuto and Mimosa's sense of "family" is something of a sham if Mimosa is ready to indulge Shuuto's potential sexual cravings (however reluctantly), and he mocks her for this (ch. 8, p. 12-17). Shuuto's corrupted her mind, as he says.