First of all, Kang Kyung Joon is a horrible and egoist person. I get it that Mari was problematic and obsessed with him, but she never forced him to love her. She did everything she could to help him like a friend would, and yet, he treated her like she was a piece of garbage. Told her to forget him, that he never felt ANYTHING for her, not even gratitude. I wanted to punch him so hard, that was SO MEAN.
Then you have a 30 year old woman dating an underage student, wow, that's really nice. Here you can see where Goblin's writer came up with the idea. (also, I just noticed that the same actor is in both dramas wth LMAO) And... What was that ending??????????? I wanted to know Dr. Seo's answer or, at least, what the hell happened to him. They basically deleted a character because he wasn't necessary anymore. This was honestly so bad. They didn't even gave him a chance to appear again to tell Kyung Joon to "fuck off, I save your life and you steal my fiancé". I thought Gong Yoo only did good roles, I was wrong. What a mess. Even though it was not his acting that ruined the show, his characters certainly did.