Annie had sensed Peter leaving. That, at first, is what had gotten her attention. She'd looked up from her drawing, lips pulled down into a frown at his abrupt departure. When he'd returned a little later, she'd gone from her window (where she'd been keeping a vigil, waiting for him to come back) back to her drawing and continued as if she'd never been interrupted.
Then she felt, more than heard, Nathan's thoughts toward his brother, and Peter's reaction, and she was on her feet and heading down to the lobby so quickly that she wasn't actually running so much as gliding a few inches off the carpet.
"No!" she stated loudly as she appeared at the second story railing. Glaring down at Nathan, her small hands were clenched into fists and her eyes were narrowed in anger. She was down the stairs and shoving past Hiro a second later, stepping in front of Peter and facing Nathan with a full-on glare. The weapons in the cabinet along the far wall began to rattle and the chairs began banging loudly against the desks they were positioned in front of as her upset grew.
"Brother!" Annie added vehemently, one small foot stomping for emphasis. "Bad. Brother!" And the glass in the weapons cabinet abruptly shattered and two of the chairs collapsed into nothing but splinters as she continued to glare at Nathan, determined to keep him from upsetting Peter anymore than he already had.