Chara is also very alarmed, although their alarmed state tends to resemble a cat puffing up.

"Hold still," they hiss, thinking they're throwing his own words back at him. "This won't take long."

And then they lunge, racing forwards for a stab.

They know he has some relation to Sans, and therefore are fully expecting to MISS again. That's why the stab is a feint, and they turn it into swipe at a location they think it's likely for him to appear.

They could be entirely wrong, but they're burning within.