This is what Park Soo Ha had to say about Jjang Hae Song "A person who is unbelievably worldly, with no manners and totally lacking in humility, there is such a person. That kind of a person started to fight for the truth, and started to see people for who they really are. Also, the appearance of that person became my light and path, when I was in the dark. If that person hadn't been there I probably wouldn't have been able to come this far. Because of that person I found out how important it is to protect someone. Because of that person I learnt how important it is to listen to a person's story."
I think all of the characters were written the way they are to compare, contrast and give voice to different perspectives and how most of the people in the world are in someway. She isn't perfect by a long shot but she grows and changes over time. Nothing dramatic and she, like us all don't all suddenly become perfect, it takes hardship, challenges and time for true, genuine, good growth to occur.
Okay, stopping now. FYI, Lee Jong Suk's other dramas (the one's I've seen) have a similar moral tale to them. Go ahead and watch it, take a step back and be open to all the characters, and their quirks, and let me know what you learned in the end.