Gao Xi is mentioned in cast like Gu Wei's ex-gf but I suppose she isn't, it's just what she wants and she using her father position and ML mother for her purpose ... my understanding after 6 ep :)
My theory: I guess Hee-Do suffers from a K-drama level amnesia, that made her forget her memories from childhood…
in a previous episode, when Hee Do chat with her mom, in present days, her mom told her that she meet Yi Jin a month ago and Hee Do seems uncomfortable hearing about him, like she dislike about it, so ...
poor you!