Disclaimer: It's my head fanon that Ozai and Lu Ten were born on the same day. Basically, everything else is someone else's.
Also, my 100th story. Most of it is pre-written, and it's not intended to be more than four or five chapters, but ideas keep coming as I write, so I have no idea where this will go.
The day of Prince Lu Ten's birth was preceded by the birth of Prince Ozai. As second born to Fire Lord Azulon, Ozai was already nineteen years behind in a game of politics and favoritism that he wouldn't even be aware of for a few more years. As the only son of Crown Prince Iroh, Lu Ten was ever the apple of his father's eye. In later years, the two princes' destinies would greatly diverge. However, they begin together, on this, the twenty first day of the eighth month in the thirty-fifth year of Fire Lord's Azulon reign.
That same day marked the end of Fire Lady Ilah's life. Her husband the Fire Lord never quite forgave the second prince for that. Or at the very least, for not having the decency to die as well.
Prince Lu Ten's mother lived until he was six years old before a sudden illness took her. In her time, she'd been mother to both her son and her husband's brother. In a way, Lu Ten was more Ozai's brother than Iroh ever could be. Once he was old enough to begin to understand such things, Iroh was everything his father wanted in a son. Ozai couldn't ever be Iroh. He would never be what his father wanted. Still, he tried.
Lu Ten was everything to his father Iroh. All he need do was breathe and Iroh was pleased with him. The day he actually began bending was the day every single person who visited that day was given a demonstration.
Lu Ten played with his father and talked to him. He was read bedtime and war stories. He was taught things and loved and listened to. Ozai saw his father from a distance, in between low bows to the floor. He gave him a report of his schooling and bending once a month then was promptly dismissed. He was mostly forgotten. Unneeded.
After all, he was only the other prince. The just-in case prince. And who ever cared about the understudy?
To Be Continued...