She sees him, arm extended in a valiant attempt as realization flashes in his eyes, hears his cry as lightning springs forth and engulfs his body. Blue electricity arcs over his convulsing frame as he slams onto the ground, hands over his heart and eyes fluttering shut.
And this way, no one else has to get hurt.
She calls his name—"Zuko!"—gut wrenching and heart breaking. Already her feet are running towards him, but a streak of blue fire crosses her path and she pulls back, hearing the princess' laughter. Zuko, her heart thumps. Zuko.
Aang and their mission were the last things on her mind now; her first instinct is to try to get to him, heal him. She would lose this fight if it meant he lived. Zuko would have wanted otherwise, the self-deprecating, angsty teenager he was. She remembers what he'd told her, during the few hours to themselves on the way to the final showdown: we have to win this war.
From the corner of her eye she can see him struggling to get up, fingers scrabbling weakly at the hard ground, eyes squeezed shut in agony. She thinks of the people the Fire Nation has struck down, and decides—it ends now. It must.
The water is calling to her.
Zuko's body is surprisingly light in her arms as she gently rolls his prone form over. His chest is still, limbs limp and eyes half lidded.
She doesn't give herself time to hesitate; drawing out a stream of water and pressing it to the burnt cloth and wounded flesh. She'd always thought the blue glow of healing was calming, but now she wonders how a colour so cool could inspire the acrid smell of burnt flesh. Her hands are shaking as she manipulates the water, and for once she fears her healing just might not be enough. But she tells herself she cannot, must not, lose another person in this war, and steadies her hand, even while preparing her heart.
A tiny flicker of movement catches her gaze, and she turns to see Zuko's clenched jaw relax. His eyes open, just a little, and she heaves a sigh at the sliver of gold under the heavy lids. His gaze meets hers—unfaltering. Perhaps his stubbornness came in handy sometimes.
"Thank you, Katara."
She's never been so glad to hear his voice.
"I think I'm the one who should be thanking you." Katara smiles, weary but genuine.
Together they sit up, her hands supporting him, warm around his neck and comforting on his back, he hissing in pain and struggling to shift his weight. She hadn't realized she'd been crying until Zuko reaches out and places a hand on her face, thumb resting just below her eye, and gently brushes a tear across the curve of her cheek. It's been a long time since she'd allowed anyone to touch her so intimately, but hey, it's a new beginning, isn't it?
It's a day of many firsts, including a kiss: right there, in the middle of a battleground and beneath the bloody sky, oblivious, just for a little while, to his sister's howls and blue fire.