I don't really have a "worst actor". I guess there are a few characters I don't like, but the actors play so many different roles. So I will change this prompt to least favorite male character, and do the same for least favorite female character. Okay here is my least favorite male character.
He cheated on Ji-Hyun and conspired with her best friend, In Jung to get her fortune. You can't get much lower than that. What's funny about him though, is he can't make up his mind. He falls for his fiance all over again without even knowing it, and still tries to string In Jung along. Seriously the worst male character I have ever seen in a Kdrama. It was frustrating to watch him act like the good fiance while he continued to try to get Ji Hyun's money and take advantage of her parents' grief.