Well, without having listened to the Rumble Fish OST in New Gisaeng Story specifically, we can assume that it was composed to complement the narrative. It might have a variety of musical styles within it. For example, there could be slow, ballad - like pieces for the romantic or sad parts of the story, and then more energetic tracks for the action or tense scenes. Also, the voices in the OST could be carefully selected to match the characters' personalities, whether it's a soft - voiced singer for a gentle character or a more powerful voice for a strong - willed one.