The person in question is the newly appointed class president, top student Fang Xiao. The original class president, Zhao Chuanfang, had transferred schools due to family reasons. The study group was purely voluntary, and generally, after joining, the best few students in studies would act as group leaders, guiding some group members in discussing knowledge points and solving difficult problems.
Fang Xiao, as the best student, happened to be in the same group as Lin Fei, who was one of the poorer students, and she was also the leader of this group. As the new class president, Fang Xiao was a very serious kind of girl in both her demeanor and dealing with people, a very earnest type. She also spoke and interacted with others in a way that exuded leadership.
