Giant.
The villain played by Joo Sang Wook falls in love with his enemy's (the hero) little sister. At first he dislikes her but her innocent yet strong personality wins him over. He becomes nice and adorable around her but he's still a villain and cruel towards others. He doesn't know that she is his enemy's sister and she doesn't know that he's evil and after her brother. Her brother and Sang Wook hate each other to death.
Does the couple have a happy end or tragic end? If you want to know read the spoiler.
They have a happy end. Suprising right? They even have a cute son together.
You really need to watch it! I know that this drama has many episodes and they are just a secondary couple but they are so damn worth it! And the drama is also pretty good anyway. (I gave it 10/10)
They are also my favorite kdrama couple of all time.