The twins are born on the Autumn Equinox.
All Fire Sages agree that it is a good omen. Children born on such momentous days are surely meant for greatness. Such is the case with the nation's beloved Crown Prince, General Iroh, honourable and resourceful man. His son, Prince Lu Ten, on the other hand, while blessed with Agni's light, wasn't as fortunate as his newborn cousins; some people speculate good-naturally that when the time will come for the young Prince to become a Fire Lord, he'll be unstoppable with the help of two exeptional Generals by his side.
(Azula summons her own fire before she turns four. She masters lightning by the time she is fifteen, and her flames burn the hottest blue. Zuko, at the same age as his sister, finds the Avatar, rediscovers Sun Warriors and the last dragons, and masters the dragon's colourful flames. Lu Ten dies before he can see them both shine in full.)
The Prince's name is Zuko, and the Princess' is Azula.
Prince Ozai presents his children to Agni, and the moment the royal family is no longer on the raised dais, leaving their subjects with bright eyes and raised spirits, the talks start.
They are both healthy, beautiful children, as one of the tired-looking maids proudly proclaims for all to hear.
The twins are a blessing for the Fire Nation, loudly preaches one of the more sophisticated Generals. And those born on such a special day even more so.
All too soon, other talks are coming up; hushed and careful.
How the Prince was born when the Agni's light was dying. How his eyes are not the eyes of a bender, too light, too bright, without a spark, shining and not burning. How he was weak but stubborn, and what a miracle that is that he hasn't endangered his mother's and sister's lives.
How the Princess was born when the first Agni's rays stretched across the horizon to paint the skies in red. How she was nearly silent, but very attentive. How she was born second, and her mother hasn't had the strength to embrace her properly before falling asleep, too tired to do more than kiss her forehead.
(Several years later, those same people will wonder whether the Prince, born in the dark, has soaked up all the day's light, and whether the Princess, coming into the world with the sun, has instead soaked in the nighttime's darkness.)
"We cannot have two heirs at the same time!" bursts out one of the younger Fire Sages. Again.
Fire Sage Shyu sights. Again.
They are having the very same discussion with the very same arguments for the third time in an hour. Namely, the discussion about who should be named Prince Ozai's heir in the sacred scripts and the royal genealogy. They don't have time for this, objectively, ostensibly because Fire Lord can ask for their word of counsel at any moment, but more importantly because, in accordance with the old tradition, they only have until the next day to update the royal scrolls and not that much time left before the sunrise.
"Prince Zuko is older, is he not?" asks another sage, clearly as fed up with this discussion as Shyu is. "He was born first, and that is what we should mark in the scroll. We are here to oversee a centuries old tradition, no more, no less."
"Old tradition it may be," argues another one of his peers. "Were it some centuries ago, we would not even be having this discussion. The Princess has as much rights to be named an heir as her brother does. We cannot ignore the fact that they were born on the very same day."
"This is not about her rights. This is about the order of their birth," reminds him mildly the tired one. "We are not discussing their accession to the throne. These children are too far in line for that. We need only to determine which one of them we should consider the older sibling. And that, I believe, should be obvious."
"But can we really make that division?" wonders aloud the High Sage and all other talks quiet instantly. "They are, after all, born of the same breath and under the same light. How can we separate them so?"
"What are you suggesting, elder?" asks Shyu warily when it becomes apparent that no one is willing to do it first.
"Leave that decision to Agni," the old man answer with the same thoughtful air. "He will reveal his will in time."
"But," startles another elder. "We cannot leave the scrolls empty!"
"And we shall not," High Sage agrees. He raises, and all sages hastens to follow. "We will mark today as the day Princess Ursa gave birth to Prince Zuko and Princess Azula, heirs to Prince Ozai of the noble line of Sozin."
All around him, Fire Sages gathered for the occasion are doing their level best to mask their disbelief and shock at such a decision. They bow respectfully and quietly, and begin the necessary preparations in silence.
Shyu wonders whether life in the capital is really worth it, and whether he should ask to be stationed somewhere less pompous instead, like the Crescent Island. That is a good idea, he feels. He'll give it some time and all due consideration.
(He will not leave the Caldera for several years, serving the royal family until he no longer can. And when he does leave, he'll become the youngest Fire Sage on the Crescent Island. And even later, when the Avatar comes to speak with his last incarnation, when the child with the weight of the world on his shoulders needs help and those who swore themselves to the Avatar refuse him, Shyu know what he has to do.)