To an outsider, she's the strong one. Only a select few know the truth: that the real strength between them his.
Inside, where it matters most, Kate is fragile: driven by demons and terrified of loss. She is fragmented; held together by unhealed scars and only inches from tumbling headlong into the abyss.
He's the one that holds her back from the edge, refusing to let go. He shines a light into the darkest corners of her soul, giving her a measure of peace for the first time in years.
Castle gives her the strength to be the strong one.