Hollow
She doesn't want to live her life, seeing every moment through the eyes of her brother.
But she can't help it- this time she's done something so very stupid, and all she can see are his eyes. They watch her, vaguely disappointed, but ultimately unsurprised.
Through the haze of pain and the disturbing delusion of Byakuya's disapproval, she watches the orange-haired boy rise up and swing her zanpakuto as though he was born to do this. The twinge of envy is unavoidable, even as she fears for the souls around her. The sisters, bleeding and afraid, the stupid, brave boy with the enormous spiritual pressure.
Her own powers as a shinigami. They are completely gone. She is hollow (she laughs internally at the parallel); devoid of everything that made her Rukia Kuchiki.
But he cleaves the monster in two, ushering it's poor, fallen soul off to the Soul Society, and she has to get up again. Can't take back her beautiful sword. Can't go home. Tries not to contemplate just what her brother will think when he hears about this.
She pulls on her brave, brash, Rukia mask and ignores the pounding fear that echoes in the hollow where her powers should be.