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I have a theory that Ye-na is Jeong Hae-seong’s stepsister, see his father had the same mission and he might met Chairwoman aka Ye-na's mother during the mission and asked her about the school’s details, the four stories so on and so forth that he ended up falling in love with her. And maybe the same guy who snitched about Jeong Hae-seong being undercover student also told the chairwoman that Jeong Hae-seong’s father is also an undercover spy who is taking advantage of her to complete his operation and didn’t actually loved her, as a result she took revenge on him and possibly killed him, but she kept his child, that would explain why she treats Ye-na like that, just using her for her own perfect image and not giving her the love and attention she needs.