This is my honest opinion of the show and I know some people will disagree with me on this.
This drama had a promising start with using mythology as their main base for the characters. Fantasy dramas like this have always intrigued me. I do like all the surrounding characters as they always stole the show with their chemistry on screen. BUT the one character that I feel was written and executed poorly was our main heroine Sam Jang (Jin Seon Mi). Her character fell completely and utterly flat for me after episode 2. I had somewhat of a high expectation that she was going to be an interesting character considering that she could see demons, ghosts, spirits and whatnot and also exorcise them. I don't know how someone who's been "exorcising" demons for 25 years still be so pitifully weak, pull cheap tricks, and still be so stupidly naive about their choices and consequences. Her character does not make sense because one moment she has sharp intellect and the next she's making deals with evil demons because she has a "oh poor me, why me" mentality. To simply put it, the main heroine comes off whiny, has a HUGE victim mentality, and her character arc is just not that interesting in general. But as for all the other casts of the show, I found them more enjoyable and their background stories to being more consistent with their character growth.