A/N: This is less a story and more of an outline told in story form. I've had this idea floating around my head so long that it's actually coalescing into a full-blown tale but I'm already so bogged down with things that can't sit down and write it out. The basic idea is of a failed Avatar at some point in the past, who actually went through what Aang only risked in fighting Ozai. Also, it deals with the question: what if an Avatar had a sibling? And not just a sibling, but a twin? This plot bunny is up for adoption.
full moon bay
There is a secret, hidden deep beneath the Full Moon Bay. Its name is Xianggang.
Her name is Kari, the twin sister of the Avatar, and she knows, from a very young age, that she will be forgotten by time. It's so unsurprising that it doesn't even hurt. Of course the world will forget her in time - Kali, in addition to being the Avatar, has a personality like a typhoon, moves like an earthquake, and lives like a raging firestorm. Nothing about her is forgettable, nothing about her is unnoticeable.
Kari, in contrast, is serenity to her sister's chaos, a gentle breeze to Kali's maelstrom. She is the sensible one, the smart one, the calm one.
She is forgettable.
In time, she accepts this. There are advantages to being forgettable, she learns, and plenty of ways to turn serenity into profit. Kali rarely agrees with her methods - especially after her time with the Air Nomads - but understands that she does not have a choice in the matter.
Kali is power, but Kari is refinement. Kali is brute force, Kari is subterfuge. Kali is the Avatar, Kari is the protector.
But Kari understands something that Kali does not - cannot - and that is that the Avatar is not a force of good. Their purpose in this world is not to bring light, or to save small animals from snowstorms, or even, really, to stop the Earth Queen's purges of firebenders. Their purpose is to bring equality, balance between light and dark, good and evil - because Kari understands that without one, there cannot be the other.
She doesn't tell her sister this. Kali does her best to exist as a force of pure light, so to counter that, Kari exists as her antithesis. Kari steals the food they need to survive, kills those who stand in their way, threatens any who dare to harm her twin. Kali would reason with them, but Kari is pragmatic to a fault.
Balance, she calls it. After all, the world will forget her.
The Earth Queen Sun lives in the Great City of Xianggang. She was the bastard child of the previous Earth King and one of his Fire Nation servants, who fled to her homeland after the Earth King's wife discovered her pregnancy. When Sun manifested Earthbending in the Fire Nation city of Hamakita, she was ostracized, hated, and - so the stories say - abused. Here, Sun developed a loathing for fire and all of its children. She vowed to one day wreak vengeance upon them.
At the ripe old age of sixteen, Sun traveled to the Earth King's court, played all of her cards right, danced with all the right people and flirted with all the right men, and managed to get herself into her father's bed. She did not reveal her name or heritage until the following morning, and used the information to have herself named heir to the throne. A week later, the Earth King and his wife both died of an acute attack of the knife-in-heart disease.
Once on the throne, Sun played by all the rules, watched carefully by the old Avatar Yuzu, who was nearing death. Yuzu kept her in line as well as he could, but died three years into Sun's reign.
She had sixteen years, and she used all of them to her utmost advantage. She attacked the Fire Nation swiftly and without mercy, starting - poetically - at Hamakita. It took the better part of ten years to subjugate the entire nation, but she managed it through a combination of guerrilla warfare and a total lack of regard for the common laws of warfare. Every firebender, from venerated old master to toddler playing with sparks was killed, with brutal efficiency. Sun did not bother to take prisoners or torture, preferring instead to cut each bender down in the swiftest way possible.
All of this completed, Sun retired to Xianggang, unconcerned with the other nations. Every so often, however, she sent - and still sends - waves of her Elite Earthbenders to wipe out any whispers of rebellion, and kill any new firebenders who may have manifested power.
Sun lives only for revenge.
Kari cries when her sister is told that she is the Avatar, and so does Kali. Why? they ask, why Kali? Later, Kari will understand, but Kali never will.
Kali is forced into the jungle to learn Firebending, from the dragons who still linger on the fringes of the world. It's a long and arduous task, and she takes to it like a fish to dancing. Kari watches her with envy and pity, and strikes up a friendship with one of the Sun Warriors, a handsome young man who calls himself Itzli. He goes with them when they leave.
Retaking the Fire Nation from Sun's clutches will not be a simple job, and none of them relish the thought of facing it. All of the royal family is dead and the country lives in a state of terrified stasis, unwilling or unable to fight back against Sun's men. Somehow, through her ferocious and dishonorable methods, Sun has broken the Fire Nation, by breaking its faith in itself. Kali means to inspire them, and Itzli still believes in them, but Kari is more reserved.
Sun will fight with any means available, and she'll use Kali's frankness against her. If Kali wants to defeat the fundamentally practical Sun at her own game, she'll need Kari's pragmatism, however little she may approve of or want it.
Kali is adamant that she should learn Earthbending - after all, she insists, how could she hope to stop Sun without being a fully actualized Avatar? - but Kari refuses to venture into the Earth Kingdom to find a teacher. With three elements at their disposal, she thinks, they should be enough. Kali doesn't agree. It's all right with Kari, though, because Kali never agrees, even when she should.
It's one of the ways they strengthen each other.
It takes three years to retake the Fire Nation, and in that time, Kari has watched Itzli fall in love with her sister, her sister fall in love with fire, and an entire nation fall in love with freedom. Kari, meanwhile, doesn't understand love.
Sun, predictably, fights back. She comes to the shore with a fleet of Earthbenders, and threatens to sink every island of the Fire Nation into the ocean unless the Avatar goes peacefully with her fleet back to Xianggang. Kali and Itzli discuss whether or not she's bluffing. Kari sits still and knows better.
Kari doesn't understand love, but she does understand her sister, and she does understand Sun. Kali is an unstoppable force, Sun is an immovable object. If they crash into each other, the whole world will be caught in the pandemonium, and too many people will die.
Kali is the idealist, but Kari is the pragmatist.
And really, at the core, they're twins.
When Kali catches up with Sun, she's already locked herself up in Xianggang, and there - for all the world to see - is Kari's head, stuck on a spear and hanging from the wall. Something within her is broken, bleeding out from her twin's neck on the high walls of the Great City. She does not transcend into the Avatar State. She does not need to.
The stories will say that the Avatar Spirit, enraged with the Avatar's murder - for the world never knew there were two - brought in a tsunami from the ocean and a typhoon from the air and a comet from the heavens, and laid waste to Sun and her city.
In reality, Kali proves that her sister was right all along. Three is enough.
The entire city is destroyed, drowned and burned and swept off the map, and then sunk deep into an unnatural bay. None are left alive.
After the destruction of Xianggang, Avatar Kali abandons the world and goes north until she cannot go north any longer, then lays down at the pole and sleeps.
Kali will be remembered as the Destroyer. Kari will not be remembered at all.