Arrange the actions of the fake Yu Xi in chronological order.
First, her initial step in the plan was to come to Jask's side. She must have deceived Jask about something, causing him to believe something incorrect, and helped her achieve part of her goal.
Lin Xian didn't know the intricacies of this relationship, only Jask did.
However, it's worth mentioning that the fake Yu Xi, while with Jask, had already begun to deliberately impersonate the real Yu Xi.
The secretary had said, the blue-eyed girl was expressionless, her gaze cold and penetrating, black hair cut straight at the neck... Clearly, the fake Yu Xi had started to pose as the real Yu Xi from that time.
She must have long been aware of the real Yu Xi's existence, knew that the real Yu Xi would come to kill her, and didn't want Jask to discover the switch between the real and fake Yu Xi later on, so the fake Yu Xi had already started throwing smoke bombs while she was with Jask.
Afterward.