Sang-Hee only rarely speaks of her life before The Imperial Gwishin Bureau. The story that has been cobbled together by its members is that she was born into the Yangban caste and something occurred during her teenage years that resulted in a curse. To avoid the effects of the curse, she wears a mask at all times around others. The precise nature of the curse isn't a matter of public knowledge.
Overall, Sang-Hee is an organized leader, taking pride in the efficiency and effectiveness of her Bureau and its members. She takes her job seriously and treats the members of the Bureau as her family.