The flashback scene really helped solidify things for how Koyun is how she is. No doubt Manatsu is pretty terrible all things considered. There are people that will act as your friend only when it benefits them. With that being said, it makes sense why Koyun says she isn’t as kind as Miki because from your viewpoint she used the relationship with Igarashi to get back at Manatsu. While she deserved it, you can understand the guilt Koyun has.
With that being said, based on the interactions that Akine has with Momoka and Minato she does not give the same vibe as her sister. Her sister put herself in other people’s business while Akine feels like doing that is a pain in the ass.
The real one suffering here is Minato. It’s wonderful to see how he and his relationship with Koyun have evolved. Momoka inserting herself into the conversation is not only causing overthinking for Minato but is also allowing Koyun to loosen up around him.
