Bae Joohyun believed in promises and took promises to heart. She believed in promises so much that she ended up keeping a notebook which contains uncompleted promises with someone. She called that notebook 'Book of Promises'. She wanted her promises with that someone to be fulfilled and she had been keeping her faith in waiting for that person to appear.

Kwon Jiyong doesn't make promises and doesn't believe in keeping promises, not anymore. He had been waiting his promise to be fulfilled but that never happened. It was a long time ago when he actually did his last pinky promise but this promise never seemed to fulfill. After waiting for a year or two, he gave up on waiting for that promise. He had came to a conclusion that promises were meant to break. Since then, he intended to never to make anymore promises ever again until the day he saw her again...