The big difference between Yongsun and Byul lies in the way they’re living their life.To put it simply, Yong reasoned something with logical thoughts, while Byul determined her action based on her feelings. The older girl loves to plan, while the younger likes to do something impulsive. It applies to many things, including their love life. For a deeper explanation, it could be understood by the action that each of the individuals did at the start of their so called more-than-friends-but-friends affair.