maybe i am overinterpreting the story too much, but if at the end Lydia is the BigBad behind the App i wouldn`t be surprised. I mean innocent rich girl, with controlling/overshadowing parents low self-esteem and a inferiority complex. Someday she just snaps and decides if she can`t control her own life she will control everyone elses. Rope in some close friends (Min) through blackmail and money, start a pyramid scheme with more blackmail and money, make more people join, create the app to streamline things and voila, she becomes QueenBee and enjoys everyones debauchery. Then one day the MC appears and she decides that she has found a new plaything. One conveniently compromising pic later, her new "boyfriend" gets squirmy and she has frontrow seats.
And still i wouldn`t like her, bah well you have to take the good with the bad.