The main reason Asar was able to hide her true intentions and operate covertly within the Black Rose family, a nest of master actors, was her cowardly and fearful personality.
This personality made it difficult for the Black Rose family members to be too wary of her. More importantly, this cowardly personality wasn't something Asar had to fake. Before her past life's memories awakened, this was genuinely her personality; she wasn't pretending.
After her memories returned, Asar only needed to follow her past feelings to portray the weak and timid personality. Moreover, sometimes enemies can be the best teachers.
The excellent acting skills of the Black Rose family members also benefited Asar greatly, gradually transforming her into an Oscar-worthy actress.
