so, i don't really get the critiques here. the artist was trying to convey an emotionally charged concept, not worldbuild. those are two separate purposes. logical holes don't matter as long as the heart of the work lands. that's subjective of course, and it's fine if it doesn't for you, but don't judge it by a rubric it never intended to fill.
anyway! my personal take is, this hits very acutely as a longtime shut-in NEET with brain issues(tm). it might be the autism specifically? but the first half is very clear in setting kei up as this freak outsider who's rejected by everyone around her and can't understand why. that's why she built marie in the first place. BUT, and here's the really fun part: what if you look at both of them as very unreliable, flawed narrators? i'm pretty sure that was the intent. everyone here talking about what love is and isn't, and that's the point. kei never had the emotional maturity or experience to know what love is. she could only piece together the idea of it from fiction, ideals, etc, and the end is her getting trapped within that forever. it's definitely dark.
i get the feeling kei has a much longer history of isolation from other humans than we get to see here. she's not really sure what humanity itself is/means, and so her sense of ethics is incompatible with most people. it's funny. you see her in the latter half finally learning to connect with other people, but she was missing that last bit of emotional maturity and empathy to understand what marie was going through. she's very self-centered, i guess is the right word? but it's still a tragedy, even if she brought it all on herself.
all in all, i really liked this. artist is awesome in general but this work is definitely a standout, and im glad i read it. i hope they get serialized at some point ngl