Nill's feathers very visibly bristle when it becomes clear that Jason has no intention of stopping what he's doing. Her eyes narrow, and she shifts slightly so that what little of her there is will mostly block the Avox from view. Being only about 5'4", Jason probably won't have any issue seeing over Nill, but the intent is still there, the meaning obvious; she's not letting him anywhere near the Avox.

You back down, Capitol brat her demeanor seems to imply, where she has no voice to say as much herself, and would never risk it even if she did. Nill's not budging otherwise though, and she takes the slap without much more than a flinch. She continues to stare Jason down and gestures behind her at the Avox to leave. Depending on how hard the hit was there's probably already a bruise forming under her skin.