Here we go:
1. Sara and YG's ex wife worked at his company. Sara was the first to have a crush on him but he fell for his wife, who was also Sara's BFF. But continued to carry a torch for him, even after YG married her best friend. Mostly because she dreamed of being the CEO of FT Group's wife, not exactly because she loved YG.
2. Somewhere in the middle, while YG's wife was still alive, Sara got a boyfriend and was planning to marry. We don't know what happened but she ended up breaking up with him and having his child. She was about to throw him away but instead put him in front of YG and her Bff's house so he would have a "good life". Since her BFF was the lady of the house, she got a job as the house manager. Once BFF died, she continued pursuing her dream to become FT Group's chairman's wife. Complete sleazebag? YES.
then she became an obsessed deranged psycho and tries to trick YG into believing that he was engaged to her when he loses his memory. all sorts of stuff happens, she mistreats YG's kids and so he kicks her out of the house. Every time YG yells at her and her evil plans go wrong, she gets drunk and calls DD's uncle, Cha Gun(he rescues her from suicide once). CG, like an idiot, falls in love with her and pursues her. After getting kicked out of the house, he continues to pursue her and she agrees to date him. While all this is happening, Sejong's biological father, who is a jerk just like his ex Sara, finds out he has a son and starts to threaten Sara because he wants the kid(don't ask me why). Cha Gun saves her from him multiple times, develops a hero complex, and also finds out she has a son. But Sara tells him her son is dead and he shouldn't date a bad woman like him. So he's heartbroken and continues trying to pursue her. Sara again starts to develop an obsession towards Seeing and spies on him and DD, who comes to pick him up from kindergarten.
That's the entire Sara arc till now.
