Sang Hoon is the protagonist of this film, he is deeply affected by his past due to his fathers domestic abuse which in turn lead to his mother and sisters death. The film, at first, has a dark comedy feel to it, mainly due to Sang Hoons constant cursing and random outbursts of violence. Sang Hoon literally beats people up to live, in more than one sense. Sang Hoon meets Yeon-Hue, a high schooler, when he spits on her uniform, Sang Hoon was surprised when Yeon-Hue called him back and stood up for herself, likely due to his experiences with domestic abuse.
Sang Hoon and Yeon-Hue spend more and more time with each other, as she gets to know Sang Hoon she grows to think of him as a father, her desire for a father figure likely comes from her own father being mentally ill and you guessed it, violent.
I really don't want write more about this wonderful film, I really encourage you to watch it for yourselves. If i could compare it to another film it would be American History X as the story lines are very similar, so much so that i got a little deja vu.