Okkkk I finally figured out what is happening here.
Ichika was gay, she did something and her mother got rid of her to start a new family, leaving here in an “inaka” town. The mermaid is kinda this freedom monster (no other word for it). Ichika is legitimately suicidal, the mermaid comes and saves her by helping her come out.
She develops a relationship with the mermaid.
Satsuki is interested in going out to the real world, the neighboring town. You can tell that Ichika is not really into leaving the town but Satsuki really wants to go. Chitose boards the bus and tells satsuki not to leave, and Ichika of course agrees letting Satsuki know it’s best where they are. This is because bigots exist outside the city.
Now we come to Haruna. She seems to come from a town very close to chitose’s town. The people there call her a new name and that saying it is dangerous to just ignore it. But back in chitose’s town Ichika and Satsuki actually interact with her.
Haruna is spiraling because her community is imposing rules and such, it causes Haruna to slowly turn into a monster because she’s basically suffocating. Chitose could have taken her to her bubble town but because society had a strangle on her, she became a closeted self loathing, thing.
So yes Chitose is terrifying but also she’s helping some of the girls. It’s mentioned that some are in-between mermaid and human and Id need to read more to understand what that means but I think this is basically the jist of the manga.