I know what you mean, NANA does the same thing to me. It's such a car wreck and you know the ending is horrible, but I love it and can't stop reading anyway. I've wanted to yell at the characters for being stupid on many an occasion.

I used to really dislike Hachi too, especially in the beginning. She's pretty much everything in a girl I hate. The very first chapter with her backstory was really hard for me to get through, because I thought it was so stupid. But later on, she began to grow on me and now I like her.