I want to hate the brother for his actions, but it's hard to, when everything was because of his illness, that wasn't treated and diagnosed before, since his parents didn't want anyone to take notice nor to make it go on his history record. He's definitely not someone to like, but before everything started he just acted pretty much like a normal teen. The things he did during the plot of this story were horrible, I wouldn't wish for anyone to go through that, neither as a perpetrator nor the victim.
I guess the mother also had her reasons for the things she did, generational trauma and all, but I can't sympathize with her at all, she's mostly to blame for everything. Father is to blame too, he wasn't there when Dahye needed help, he even hurt his family multiple times. He got redeemed a little during the second half, so I can't really say that he was worse than the mother. Hard to say anything about grandma since she hadn't had so many scenes, but I can only guess that she had some personality issues as well.
I really liked Dahye, she was really pretty and a bit of a potty mouth. Her friend, auntie and guy that she dated were amazing too. Some people may think that the redhead guy was too much of a convenient plot device, but he was a walking green flag in my opinion and pleasing to the eye, so I'm glad that in the end Dahye found her happiness.