When Zuko is a child, he tells the world that he will become the sun when he grows up. Mother laughs, and even Father praises his ambition. Yes, Zuko knows, he will be the sun.
When he is a little older, he no longer says it, but still he knows the truth. But fire will not come to him, and this frustrates him. It should, it must, he can feel it burning within! But it will not come to his hands.
Not until he wakes in the night to the feeling of fire-in-flesh that is a bender in his room.
He takes a breath to demand an answer, and the man thrusts a pillow over his face.
Indignant rage boils within —how dare this man strike against him— and he slashes a hand at the air, screaming that fury into the cloth, and the fire within flares a blazing arc from his fingertips, searing the mans throat out.
There is praise now, praise for his bending, but though flame will now come to his hands, he knows with clarity he must not summon the white blaze, not until he finds-
But the thought will go no further, and the effort to contain the white blaze makes everything difficult, his bending slow, his motion clumsy.
Years pass, and the feel of something missing grows, something he has to find to know- what? And to balance him out, something that counterweighs the white fire.
Then comes the banishment, and while he heals on the ship, he wonders if the Avatar is the answers he's seeking.
At last, he has the freedom to search.
Years pass, and he cannot find it, not the Avatar, and not his answer and counterweight. Frustrated, he grows more impatient, restless, knowing the answer is somewhere, but never in reach.
The Avatar is twelve years old, and he is not the answer. Zuko knows. The Avatar is important, related, and he is glad the boy lives, but his frustration grows. Years now he's told himself the Avatar is the answer, and now- but he is important, and so are the Water Tribe children with him.
He follows at a distance, never close enough to catch them, never far enough to lose them, and fights by their side at the North Pole, finding the Avatar meditating where the moon spirit is threatened.
Zhao must not kill the moon!
He throws himself before the gout of searing fire without a second thought, and wakes to the touch of water on his face.
The waterbender has healed him. She knows it; he can see it in her eyes. She too is looking for the Answer, and he is part of it.
He travels with Avatar Aang now, searching for an earthbending teacher and the Answer, the Answer to a question he hand the siblings share.
And the girl, Katara is the counterweight. He knows that now he can use the white blaze.
It's in Gaoling they find the Answer, in the form of a blind earthbender who raises her head and turns to them, waiting for them after her victory.
As Aang asks her to teach him, the Answer dawns on Zuko, the siblings, and the blind bandit as his sun dawns on the sky.
"Oh," says Agni in sudden understanding, Tui and Guanyin echoing it as Tenigri loses his grip on his dinner.
With the World Spirit frozen, the balance had grown unstable, and the four of them had chosen to enter the world as the Avatar did, both to find him and to provide stability to that which they governed, his sun and the blood of the planet, Tui's waters and chill which countered his heat, Guanyin to quell the tremors in her earth, and Tenigri to calm the hurricanes.
After this it is almost too easy.
Azula may have been a match for him when he was hampered, but now, in balance with his opposite, she has no chance, and he withdraws his blessing from her, regretting his inability to kill, but remembering the consequences any of them would face upon tainting their power with willful death.
They teach the Avatar what he needs, not to fight, but to restore the harm his absence has caused, and make their way to Ozai's palace.
It is time to end this.
None of them may kill him, but they do not need to. Their confinement to these bodies may mean they have to stand before him to withdraw all blessings, but they are still gods bound into mortal form, not human benders.
The other three lament that with their human knowledge comes the fact that casing him away from what they govern would be death, and Agni/Zuko nod in understanding.
Before, it would still have been death, but they had not understood, and it would not have tainted their power. Now is another matter.
They leave his judgement for the rulers of the other nations, and travel with Aang to restore their world. Sokka bestows his blessing freely, and new-made airbenders flock to Aang for teaching. Tui does likewise, until water stands equal to air and earth, while he withdraws his fires and harmony is restored.
It is not so bad, this human life, and the world is still fragile.
A decision is reached. They shall remain until these bodies die, and only then return to the other realm.