Neriii

Very quick aside about her word choice here: she has a tendency to call things stupid when she wants to downplay that it matters to her:

There's another usage I'll bring up later, but we see the pattern here.

So, we get to the festival question itself.

We get that little "uh" in the first bit, just a little note that she's not saying this with full confidence. We see more heavily in other conversations that she's kinda prone to using uh/um/like filler words when anxious to bring up a topic (biggest example: diving in Kris and Noelle's history in Dess's room). The other note here is that pause. Like, why pause there. There's only really two options that would be on her mind, right? Like, there's the whole idea of being dramatic/building suspense for an option, but I can't see it as that. Maybe if Noelle was last, since she's a more recent friend to Susie and she doesn't know yet that Kris and Noelle have any history, but Ralsei? Like, their mutual bestie/trio member is the most obvious person for her to bring up. We'll come back to this. So, the choice comes up and we get the options that include "You," which we know is the one Kris wants. As for Susie's reaction:

Now, let's dissect this a little. First panel: Essentially speechless, just a stuttered "huh?" This is the part I'll focus on the most Second: Accusation of dodging the question. Note that like, it's not just a little joking way of saying it. Fully bared teeth, double !!, kind of an extreme reaction Third: One of her most significant lines to Kris, her trying to state it's obvious they'd be together. Main thing to note here: She's sweating. She's saying this anxiously, not just confidently. We can see that from the pause before it too. Fourth: Pushing for that little affirmation, wanting to be sure Kris agrees. She has a tendency to probe Kris for agreement a lot. Ok, so this first one is my focus here: Pursed lips, speechless stuttering. This isn't the first time we see this type of reaction from Susie after a choice. In fact, we see it three other times: two earlier times in Chapter 2, one later time in Chapter 4.

First time is if we choose her for the gift:

Rendered essentially speechless, same pursed lips expression. Eyes wide in her walksprite, full on shock reaction.

Too long of a follow-up convo to post the whole thing, but after she guesses that Kris stole it, she calls doing so "stupid" twice before trying to steal something for Kris in return:

(note again the bit about calling things stupid when she wants to downplay them) Key point: It catches her off guard, but is a very positive reaction. Second time: If you go to Kris's house during the Chapter 2 Epilogue, but don't choose to go home yet:

Caught completely off-guard by just the idea that Kris would want to spend more time with her. The option she wanted, but she still wasn't expecting it. We again see a very positive response from her, the first time we see the Susie triple affirmation.

The latest, and most significant time we see this response: The Chapter 4 piano story. This is the most emotional vulnerable we see Susie get. She tells the story about smashing the plaza piano. A very shameful act directly associated with Kris's talent. There's so much that could be gone into about this story (It's imo the best written Susie scene and my #3 scene in the game) but lets get to the choice after.

On the lead in to the choice, she's already regretting even telling that story. In that second line, she's already speaking like she expects a negative reaction. She not only fears but instinctively expects rejection, even when it's from someone she sees as very close to her. We see it demonstrated in interactions with Lancer and Ralsei. We see it explained why she's like this in the story with Tenna or Climbing the 2ndSanct towers. It's why she's so caught off guard by something as simple as Kris not going home yet, because her instinct is to expect the negative response. And once again, with the most positive choice we can give, "If you play too:"

Our fourth instance of speechless Susie. Head back up, eyes widened. She was not expecting that response at all, but she's so happy to have gotten it. Like with the going home bit, we get another Susie triple affirmation.

So, we return to our original question and reaction.

With all this in mind, we can infer why she makes that initial pause. She's asking this to Kris, so Susie herself being an option would make sense. But Susie is someone who not only fears, but expects rejection. She would never list herself as an option, because that would risk that option not being picked. So she pauses for a second to debate doing so, but can't do it.

But when we have Kris take that unspoken option, the one they clearly want to take, we get that speechless reaction. As we've seen from the pattern, Susie making this face while being speechless after a choice is her being surprised, but in a very positive way. Everything after that is just to deflect from how surprised she was, by treating the answer as something unspoken and obvious, despite that just before all this she's rendered speechless if we let Kris choose to spend more time with her.