If there was any list of classics in Kdramas, I would always put this one on that list. It was the first drama I gave a 10. Never say goodbye song due to which I was drawn initially to this drama.

It has most of the cliches Kdramas are famous for - Love triangle, Cohabiting, Fake-siblings, X-year timeskip etc
All of these cliches are done in a beautiful way.

Yoo Rin is a 20-something girl with having a pile of debt which she owes to some thugs. Running from such people is how she meets Gong Chan. Gong Chan helps her unwittingly, that's how this story starts.

One nice thing I found about this drama is Gong Chan and his grandfather's dynamic. Even when grandfather betrays his trust, he still manages to do his filial duty. Usually, in modern dramas a ML/FL if is betrayed by parental figure, they resort to (though temporarily) rebellion almost immediately.