Zitong was a pet name, probably because the two words "Zi" and "Zitong" both implied beauty and beautiful wishes. In ancient China," Zi " was an important wood and was often used to make it, while " Tong " was a kind of fruit that tasted sweet. Therefore, in ancient literature, people used " Zi Tong " to describe those beautiful, precious, and desirable things. Therefore, the male protagonist calling the female protagonist "Zitong" might be expressing his praise and wishes for the female protagonist's good things, or it might be a metaphor for their relationship. However, it should be noted that this is only a personal guess. The specific reason may vary according to the plot background and personal preferences.