I loved the two main characters and I felt invested in their outcome. The reason I lowered the score has to be the directing.

A lot of the shots are muted, almost as though gauze has been added to the screen and like the opening scenes of shadows the story is not too clear.
Yes there is an easily understood revenge from a girl who was heart broken when her father left the family for another woman. Her pain is exacerbated when she tries a confrontation at his funeral and then, as details are revealed, she becomes obsessed with payback revenge.
Unfortunately her target is the woman's son and the story gives a good build up to how he is being targeted by her and a very viciously jealous ex boss.

The complex cultural need to lump a families wrong doing onto other members of the family is hard for me to fathom. The gap of a year before her sister lets drop the information that the man our FL has fallen in love with has actually been accepted by her mother beggars belief. On that note why does the ML also not share this gem of information but instead wanders off for a year?
I was exhausted by the last episode as the director dragged out the slo-mo movements and ML leaving then coming back with flowers. It felt too contrived and less logical.