I thought Hinako already knew Miko's true form and her previous man eating habit?
She knows that she ate humans before. But not about the brutality of it. She just thinks she eats because she needs to like the mermaid. But Miko was somewhat more evil than that because even if she does not need to she eats humans cruelly. That's why she asked the mermaid before how she ate humans and was taken back on how she responded because their ways are different.
So Mian knew it all along?
i was surprised too at first but then i remembered if you go to first chapter you can see clearly that in a car ride haegu tells child mian that her father killed her parents
yes