Here's what I've written so far. It's a first draft, so I may have to change some later. I have some more planned out, but I figured a flashback might be a good start, and I added the line about her doing anything she set her mind to because I remember that from her introduction, and I thought it might be a nice parallel.
Having barely escaped his fate, Kin slumped down against the warm, scaly body of Nagendra, his bodily scars throbbing with pain. The Avatar had torn off a piece of his already-torn pant leg which he now wrapped about his arm as a bandage. As much as he felt his body aching and squirming after being beaten to within an inch of his life by one of those most precious to him, it was nothing compared to the further torment the train of thought tunneling and searing through his mind bestowed upon him. "She's still alive?" His eyes drifted shut and his mind instantly wandered to the four neatly kept graves at his house, the outer left tombstone being that of his beloved daughter Sakura. "How can that be?" Kin uttered to himself aloud, part of him still trying to deny the startling revelation. He remembered Sakura by his side during her early firebending lessons.
The curious little girl looked on in awe as her father twisted his arms and spun the elegant display of flames around in a fiery dance. He proceeded to condense it all together into a rotating red-hot sphere and whipped his ball of fire back and forth between each hand, and then over his head. Then, with another twist of his wrists, he extinguished the flame and turned to his awed daughter. "Now you try," he instructed her calmly.
Sakura stared up at him with her golden eyes, baffled that he expected her to perform a routine like that so soon. Nevertheless, she returned to her father a resolved nod and rooted both her feet in the earth, her little arms and hands mirroring Kin's motions. A flurry a red flames appeared and echoed her own motions. Sakura then shifted her weight to the side and the burning strings wrapped around one another one at a time to form a loose, but definite, ball of fire.
"I did it!" exclaimed Sakura. "Daddy, look!"
"Very good," Avatar Kin acknowledged, smiling. "You catch on very quick. Now, try this," he added as he whipped up another ball of fire, then bent both knees and thrust his arms in front of him, shooting the blazing orb toward the nearby woods and through a distant space between two tree branches.
Determined, Sakura stabilized her still-spinning fire ball and jabbed both her slender arms like her father had done. Sakura's fire ball jetted toward the same tree, but lost some of its steam along the way. It shrank in the last few feet before it reached one of the larger branches about four feet beneath the open space her father had aimed for. The wooden arm burst aflame and the fire rapidly snaked up along the rest of the tree. Sakura gazed up at her father with a guilty grit of her teeth. "Oops."
Fortunately for the tree, Avatar Kin had sprung right into action and snuffed out the flames, limiting the damage to the one charred branch and a small portion of the trunk around it.
"I didn't mean to destroy that tree," Sukara's voice was loaded with the promise of tears. Kin knelt down beside her and gently placed his hands on her shoulders, gripping her small frame.
"Don't worry about it, sweetheart," he reassured her, "it's okay." He lovingly gazed at his sniveling daughter, who slanted her head in defeat. Kin could not help but smile; she was still so full of innocent care for the world around her. "That is just fire for you. It's an energetic, stubborn element. Sometimes, it has a will of its own, but you'll learn to control it. Don't give up. The tree will grow back." He lifted her chin to allow their golden eyes to meet. "You can do anything you set your mind to."
He remembered the joyful fire in her golden eyes when she performed awesome feats with her firebending with him for the first time. Her merry laughter resonating through his memories morphed into an anger-fueled growl and his little, giddy girl changed to the young woman he had faced earlier. She was so different than he remembered, but she was definitely his little girl. Those shiny golden spheres were undoubtedly the same as the ones he had looked into years earlier, thought they were no longer as he remembered. They had grown colder, harder, more aggressive; they were devoid of innocence. You can do anything. The words ricocheted through his being as he thought of his daughter, now alive again and grown up. You can do anything. And indeed she had. Kin always knew that she could, and she had been only six then. You can do anything.
"What have you done, Sakura?" The words floated past his lips like a soft whisper, though his mind was preoccupied with another question, one that he was afraid to say out loud, fearing that it might be the more accurate one, though the guilt of that would be too much for his beaten person to bear. What have I done, Sakura?
He did not want to remember the day he had come home to that empty house years ago, but it was chiseled in his being as a permanent mark of his ultimate failure. He carried it with him, wherever he went, whatever he did. All these years, he had thought her dead, just like her mother and her brothers. It hurt. He was hurt. His body remembered the pain Sakura put him though and a light sparked in his mind. She was hurt, too.
The light pierced the shadows threatening to succumb him. Kin groaned as he tried to stand up. Nagendra offered him his head for support and pulled him up. While gently stroking his dragon's head, Kin was at a loss of how to confront the issue that stood before him. Communing with his past lives would not bring the answers he sought, as this was more a personal issue than a worldly one. No Avatar would be able to offer solace. I sent that feathery spirit back to his own world where he belongs. Sakura's words came to him as thunder and his eyes widened with realization. "Wan Shi Tong. She met him... She must be at his library and sent him back to the Spirit World!" With tremendous effort, he dragged his body onto Nagendra. "Come on, boy, we need to get to the Eastern Air Temple. It's the best place to meditate into the Spirit World."
As Nagendra's strong wings took him high into the sky, Kin thought about the challenge laying ahead of him. Seeking assistance of the Knowledge Spirit was a troublesome notion as it would require him to present a piece of knowledge to Wan Shi Tong, and he had all but shed his possessions of the physical world since his own family departed it. However, he did know of one thing: now that he knew Sakura was alive, he would do whatever it took to find out what happened to her and how he could set things right. If Wan Shi Tong was the one who had the answers, Wan Shi Tong was the one who would help him, whether the spirit liked it or not. Resolve wrapped itself around his battered form and seemingly lifted his pain. I lost you once, Sakura, I'm not doing it again.