Soomin was horrible to Jiwon from start to finish. She was a clingy psychotic parasite to her "other half". She was a bad friend with no limits. She betrayed her only best friend. 

But is that all there is to the relationship? Until her death, Jiwon had both tolerated and even enabled Soomin's behavior & attitude. Near the end of the story it is revealed that Jiwon has always known the painful truth of Soomin's past (and their shared past). 

Despite her meek nature, Jiwon has always had a core of self-confidence due to her relationship with her father. Jiwon didn't have to wonder what it feels like to be loved. She just had to learn to stand up for herself and stop being a pushover. Soomin never had a loving parent like Jiwon's dad. She didn't grow up knowing what being loved feels like. She needed help to learn how to move past her hurts, love herself, & trust others.

After her rebirth, Jiwon separates from Soomin (and Minhwan) and begins to establish herself as an individual. She begins to assert herself at work, and even explores new friendships. For a while Jiwon believes her fate will shift to Soomin and is willing sacrifice her friend in her own place. And even though that never occurs, Soomin is never once offered a way out, an end to the madness, a moment of healing, a helping hand. Why doesn't Jiwon do more to help Soomin? I do not recall ever seeing Jiwon ever speak a word of forgiveness or feel empathy or consider an act of compassion.

Should Jiwon feel any guilt or remorse for not doing more to help Soomin heal? Can the excuse of eluding an inevitable fate persist, or does Jiwon still have some inner work to do vis-a-vis confronting her culpability and regrets?

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A sensitive soul like Jiwon will eventually regret that she didn't do more to help her "sister", whom she enabled for a lifetime before her own rebirth & transformation. Even the pushover in a dysfunctional relationship is part of the problem. Another better answer exists had she not settled for revenge.

I wont go into details because it will take forever, but soomin is a narcissist. you cannot help or heal a narcissist. I think the last scene in prison means that even after hitting rock bottom, she still lies and still wants to destroy jiwon. she lost everything, she tried to kill jiwon, and still, she says that she is her only and best friend. 

she is not willing to learn from her mistakes. she doesn't even think she made a mistake. and she never genuinely ask for help. and that's very consistent with narcissism. narcissists always think that they are right and all the world is wrong. they don't want to change, that's why it is quite impossible to help/heal them.

but,

I agree with you, this never occur in jiwon's mind, and it would have been a good idea to explore this side of her and of soomin's personality.

lastly, becoming a narcissist isn't something that happens to you. it's a choice. you choose constantly to hurt people in order to save yourself. everybody face hard time in life. not everybody becomes a narcissist. you become a narcissist when you choose to blame everything except yourself for the things that happens to you.

this level of personality, everybody have it, around 5 yo of age. but then, normal people grow up. narcissists choose to stay immature and to let other people coop with their non sens. and at this point, enablers enter in the picture: it's BECAUSE narcissists find enablers that they grow worse and worse in time. minwah because of his mother, soomin because of jiwon.

the only way you can help a narcissist is to let him/her down. and sure, they'll throw a hell of a tantrum....

Interesting!  I hadn’t considered whether Soomin was a narcissist but that rings true to me too.  I did think about whether I (in the place of Jiwon lol) could find some compassion for Soomin.  But I determined that I couldn’t because Soomin never showed a bit of sincere remorse or humility or really self-awareness so it would be tough to muster up any positive feelings for her without feeling like a doormat.   And from Jiwon’s perspective I think Soomin put on a strong, happy face and never showed any vulnerability when they were younger. So as far as jiwon knew, Soomin was doing ok despite her circumstances. ?‍♀️. 

why should Jiwon offer her any words of forgiveness??? Su Min slept with her husband and killed her, willingly. And even before she did that Su Min was the main source of Jiwon's misery in high school, from turning the school on her leading to her being bullied to Eun ho, her then crush, hating her??? and Su Min played innocent the whole time. Jiwon was nothing but good to this chick and all she got was a world of hurt and ultimately death. Sympathize with her???? For what, she never made Jiwon aware of how she felt about her father leaving her and Jiwon believed that Sumin believed that he was dead. And if I was in Jiwon's shoe and was given an option to live with the condition that Sumin and/or Minhwan dead, I'd jump to it cuz they both betrayed and murdered her.