Eun Yi-Soo's possible personality disorder?

I think Eun Yi-Soo did it too.  She seems so off. That whole family is bizarre.  The mom isn't mentally healthy either.

It would be a pretty big head-fake.

The way the older sister was portrayed makes me think she is the kind of person who would commit suicide. 

My take:
She liked Hwan-ki (after initially liking Woo-il - paralleling with the younger sister) but ended up sleeping with a drunk Woo-il.
Hwan-ki finding out makes her feel even more shame, regret and embarrassment.
She hears Woo-il blame her for the sex and 'relationship' and she's worried she'll become the scape-goat. 
She finds out she betrayed and hurt Hwan-ki's sister which compounds the problem further. 

Essentially she doesn't know how she will face Hwan-ki and her family. I guess it's possible she had a conversation with Yi-soo and maybe Yi-soo pushed her over the edge. There might be other scenes with other people too that lead her to commit suicide (maybe even involving Hwan-ki).

As a bit of an aside: the actress playing the older sister did an amazing job. I like the expressive face. I want to see her and the guy playing Hwan-ki as leads together in a drama.

@naksken I think it's interesting because the opening scene of the drama does show her stepping out of her shoes and jumping, but they show her jumping from a roof and we later learn that she jumped from Hwan Ki's window so while I don't necessarily think that Yi Soo physically pushed her or anything, I feel like that opening scene was more like the "official" story of her death. Especially since Woo Il didn't want people knowing where she actually jumped from. 

I agree though, I definitely think Yi Soo played a part in her decision to jump because I actually took the older sister as someone quite resilient. I mean, yes she was quiet and timid but I don't think everything that happened would have been enough for her to commit suicide. I definitely think something Yi Soo said was the catalyst to her decision

I really like where this show is going, and i'm so glad they paused and did a rewrite, it's so much better than it was

@alexa what's the bet there's a ledge outside his window?

She might have some type of personality disorder. She's terrified of being abandoned by this guy, so much she runs in front of a moving car. She keeps tabs on her brother. I'm not doubting she cares about him,  but I feel like she does it to make sure he hasn't abandoned her. Also the frequent things she does to hurt herself. Those are all traits of some.

I do believe it's BPD. I think I have some slight version of it myself, and it's a LOT to take in. Sometimes I get so attached to people that it really feels like I'm going to lose myself the moment they leave me, so I try to hold on to them no matter what, even if it ends up hurting me more in the end. It's a really hard process to learn to let go of people when they no longer want to be a part of my life, and this is coming from someone who only has a mild version of it -- I don't physically hurt myself when this happens, but it's still a huge low point of my life and really suffocating.

Yi Soo definitely shows the signs of BPD and definitely deals with it in the way someone with BPD would. She gets irrationally angry and manipulating when she realizes she might be left; she makes it worse for everyone and even if SHE'S suffering too, at least Kang Woo Il wouldn't leave her and she'd still have something to hold on to. When she can't get him back, she resorts to harming herself -- because Yi Soo without Kang Woo Il feels like she can't exist.

I realize that you've posted this when things weren't yet cleared up, but you couldn't have been more RIGHT about this whole situation. Yi Soo needs to deal with her problem, and that's the best thing that can happen for everyone involved. 

Of course they made the whole family wacky; Hwan Gi has social anxiety, Yi Soo with her BPD, the father has anger issues, and the mother shows signs of histrionic personality disorder. But I think it added spice to the story. I'm going off topic here, but I hope they'll handle Yi Soo's problem the same way Hwan Gi did: therapy, counseling, and overcoming it alone, not being pushed to be someone who she isn't.