YWS has 3 modes: when she's by herself or pre-Jiuchen Valley, when she's with everyone else, when she's with Ziyu / also her currently evolving mode
1: When she's by herself or pre-Jiuchen Valley
Default look: tired of the world look/indifferent attitude
This is the original YWS. Almost nothing surprises her. Even if she is surprised, she rarely bats an eye. She has become too jaded. Even when HYS tells her she'll die either by the hands of Wufeng or Gong family if she fails, she just matter of factually says which method was less painful. This real YWS just wants one thing: get it over with and attain her freedom. This is also the start of her arc.
2. When she's with other people or when she needs information
Default look: confused
In the first few episodes, people said YWS always looked confused. For me it is deliberate. In the beginning, she always had to act like she doesn't know enough (because most of the time she really doesn't), but by making the other party/ies feel they have the upper hand, she baits them into giving her information. We often see this happen with SGQ (especially in the beginning), but it also happened with JF & GZS. She doesn't look like this when she's alone. So look out for this confused look, because it doesn't mean it's the only expression she makes, but that it is deliberate and it means she is about to gain new information (and so are we).
But there's only so much new information she can gain, especially from SGQ, so if you notice, as the story progresses, this confused look becomes less and less, and she starts to show her real personality from time to time. Even though she's still pretending to be weaker (she always tell SGQ not to overestimate her because she's just a chi and she hasn't plan things elaborately while telling SGQ that the latter has already plan everything ahead, making her think that she's the smarter of the two), at the same time YWS is purposely letting slip enough information that serves as a warning to SGQ that she's not someone she can easily mess with, and SGQ gets the message (she has commented multiple times that YWS seems too smart to be a chi plus the similar overturned teacup mannerism with HYQ). So even though SGQ knows YWS is a chi, she would never actually underestimate her. (I think that's why during the identity confirmation scene, when it seemed that YWS was about to be discovered, SGQ urge her to follow the wolf ate the other wolf plan, because deep inside she's afraid of YWS and would be glad for an opportunity to get rid of her (but this has slowly changed and is no longer true, especially based on the last episode SGQ maybe glad to have someone going through the same thing as her - the mission and the fortnight flies thing).
3. When she's with Ziyu / also her currently evolving mode
Default look: innocent, soft (at first very deliberate, but as you can see with each episode progress, it is becoming more of a genuine reaction to Ziyu's actions)
At first, YWS is very deliberate as to her approach to GZY (all her actions were aimed at luring him in). At first, that was all there is to it. She got him. Hook, line and sinker. And she knows it. But GZY isn't what she imagined. He's sincere and truly kind. We first see YWS looking at GZY with new eyes when he rushed to stand in front of her, to protect her, BEFORE her identify was confirmed. Then he just had to follow that up with the medicine and the cute pillow tuck in during the smoke fiasco. When she touched the fox tail ornament, she actually looked confused (and this time it's genuine, but we gain new information about YWS thoughts and feelings). And when she looked at GZY from where she was lying down, we know something had started to form. And we know this to be true because when they had their first long conversation, YWS ended up sharing something personal (more personal than GZY will ever guess). Somehow, when GZY is involved, YWS ends up becoming emotional and revealing something personal (either to GZY or to us the audience). And this trait is not found in her first mode (the original YWS in #1). But when she's with GZY, his actions make her remember her moments with her sister. Why? Because both care for her and when she feels GZY affection, she remembers her sister's as well. And that's why it's only with GZY that YWS gets to become her true self ("girlish" as her sister said). The YWS in front of GZY may have been originally an act, but now with each episode, as we witness YWS witness GZY's kindness and sincerity, she is also slowly responding with hers.
And truly, I'm excited to watch YWS coming to terms with her feelings (something she has set aside most of her life, probably more so when her sister died). We're still a third of the way, but judging from what we've seen so far, I don't think I have to fear about YWS killing GZY. She may even protect him with her life. And as for GZY, we don't even have to guess what lengths he'll go to in order to protect her.
