Ok Taec Yeon's acting just stole this drama, he was just simply amazing, how well he played a cute and later psycho character, just wow. Kwak Dong Yeon also stole cause his character in the end was so easy to like. Villains were annoying - which is good thing, we were not suppose to love them.
To be honest it really is a good drama, there is action, comedy, thriller, even romance and great acting but.....in my opinion to many episodes and too many, sometimes too slow (or just boring?) I think I would like it better if there were 16 episodes. And unpopular but I do not think that it needs second season.