Obligatory Disclaimer: Avatar is not mine. This was written for fun, not profit.
The war went badly.
That's the only answer Chiyu's old man ever gives - and not for lack of coaxing, because Chiyu talks and talks and talks and tries to ambush him with questions, hoping that he'll let something slip. But he's too clever for that. He was a soldier, and all that matters is that the war went badly.
Chiyu knows better than to ask whose side he was on.
They make their living delivering messages and goods and passengers between Ba Sing Se and the backwater villages, scuttling off the main roads when the Fire Nation's patrols march past. Chiyu tugs his big round hat down to shade his eyes and watches their curly-toed boots stamp past. He is smaller than most other twelve-year-olds, his brown hair tumbling out of his stubby ponytail, and he is kind and gentle and soft-hearted and should never be a threat to anyone.
But they don't see the patrols very often. His old man says the most important thing about soldiers is knowing how to avoid them.
Chiyu loves the air and the wind and the sound of rain, and he drives the delivery cart too fast along the old roads. When he was little he wanted to learn how to fly.
His old man likes to lean over and thwap him upside the head, right under his hat. "Calm down," he snaps. "You're worse than your father."
He yelps and pouts and is unsurprised - his real parents died a very long time ago - and always sticks out his tongue. "What'd you do that for?"
"Because I can." His old man considers him for a moment. "And because your father was an idiot."
There is no anger in his voice, but Chiyu steals his hat anyway and is pushed off the cart for his trouble.
His father, he suspects, would have received the exact same treatment.
His old man teaches him how to punch and kick, and then four different ways to turn a firebender's own element against them. He shows him all the easy ways to hurt someone with a carefully-concealed dagger, how to use a long blade like it's an extension of his own body.
Chiyu learns all these things easily enough - he is slight and quick, after all, and this kind of fighting suits him - and when he catches his breath, he wonders about the whys of it. He can't bend earth or water or fire - and not air, of course, there are no airbenders left - and he never wants to fight anyone. What's the point?
"The war went badly," his old man says.
Some days he tries to dream up a history for them all - for his parents and himself and his old man, who still gets lost in parts of Ba Sing Se and wears an Earth Kingdom name like a shirt that doesn't fit and knows all the ways to break out of a Fire Nation prison - but he doesn't bother very often. He will worry about these things later, when he's older. He can feel it in the ripples and currents in the air, in the way he's too light and fast for his age, like the wind is trying to lift him up.
For now there is work to be done and things to learn, because he must learn while there is still someone left to teach him. That's just the way things are. Chiyu knows that nothing lasts forever, and that maybe someday he will be the instructor.
What he could possibly teach anyone is beyond him.
And then the letters start getting thicker and the packages get bigger and there's more and more passengers heading south - "it won't help them," his old man mutters - and there are rumors of problems in the north, of the Water Tribe and its warriors beating back a Fire Nation fleet and of earthbending bandits throwing supply convoys into chaos. There are whispers of the Avatar, and Chiyu thinks he hears voices in the air.
"She'd be your age," the old man says matter-of-factly. "Younger."
Chiyu almost rolls a barrel over his foot. "Huh? Who?"
"The Avatar. She'll be helping the waterbenders."
"Why?" He drapes himself over the barrel and squints up from under his hat. "How'd you know that?"
"Because she's the Avatar," his old man says without so much as looking at him, "and the Avatar is always an idiot."
Not long after that it's suddenly against the law to talk about the Avatar or the Water Tribe or the bandits, except that everyone knows Fire Lady Azula wants them all dead. Sometimes posters turn up, with rewards and dead-or-alive all over the place, and Chiyu takes to going barefoot like the bandits and twisting his hair up in a topknot like the waterbenders' scowling leader.
His old man tells him to put his boots and hat back on and to stop being stupid.
One day the Fire Nation comes back to Ba Sing Se in earnest - hundreds of new soldiers to supplement the usual garrisons, although they must be needed up north, and he and his old man watch with the rest of the silent crowd.
"Why'd they come here?" he whispers.
His old man's good eye is narrowed and his scarred fingers are curled into fists. "They're looking for someone."
He feels like he's trying to keep his balance, lost in a storm, and just for a moment he wishes he really could fly. "Who?"
But his old man just grabs him by the wrist and hauls him back through the crowd, away from the main streets and the firebenders. He feels the wind ready to lift him up and away, great gusts twisting around his hands like could reach out and take them, and his stomach knots up in a kind of belated, mute horror.
That's all the answer he needs.
His old man dresses him for a long journey, bundling food and blankets into a pack and knotting his big-brimmed hat tightly under his chin. There is a certain kind of grim finality about it, like the last ceremony before a big battle, and that is how he knows that he's going north and that he's going alone.
But all Chiyu can think is that he's nothing special - he drives carts too fast and fights like a firebender, like his old man - and he doesn't want to hurt anyone.
"Were you hiding me?" he asks, his shoulders squared and his jaw set, and he will never know how much he looks like his mother at that moment. "Am I an airbender? Why do they want to find me? Why?"
His old man slides a knife into a sheath and shoves it into his hands.
"Because the war went badly," he says.