I have decided (rather abruptly) that this will (probably) come to be a divergent fic.
Iroh was his brother by birth alone. In the sense that mattered, Lu Ten was the brother of his heart. Lu Ten was the one who helped him put the badgerfrogs in Nanny's desk. He was the one who listened to all of his ideas to 'take over the world'. He was the one who was there when no one else was. Together, the two young princes got in and out of so much trouble together.
They were brothers in arms.
To Ozai, Iroh was just Lu Ten's dad. He held no affection for his older brother, neither did he hate him. He was as neutral to his presence as his father Azulon was to his. The only thing Ozai liked Iroh for was to teach him the Firebending techniques he'd mastered when he came home from his war tours. Otherwise, he didn't care. Sometimes, his brother would tell him and Lu Ten interesting stories, but mostly, he just went on about tea and women. Ozai, at ten, didn't particularly care for either.
Though Ozai could admit some jealousy, if only to himself, in regards to the relationship Lu Ten and Iroh shared. Just this morning, Iroh had come home. Lu Ten had immediately abandoned their game of Fire Tag and ran for his 'Daddy' with a flying tackle. Ozai couldn't imagine himself making such a display, especially in front of his father. He could practically see the disapproving glare and hear the lecture for such 'undignified exuberance'. If anything, he'd run away from his father. Young as he was, he knew that there was something not quite right with that.
If he let himself, Ozai could probably hate Lu Ten. Not only was his dad not completely terrifying, he was going to get the throne. That is, if Father ever decided to die; Azulon had always been old, as far as Ozai was concerned. It was a long way off, but it was his sure thing.
It wasn't fair that Lu Ten was going to get to be boss. Ozai was older than him by a whole day.
However, Lu Ten was his best friend; perhaps his only one. That Zhao kid from the academy was more like a willing stooge. If Ozai were to be Firelord, then both Iroh and Lu Ten would have to be removed. He didn't care so much about Iroh though. Lu Ten really had his heart set on telling everyone what to do when he grew up so he probably wouldn't just let Ozai have the crown either.
He could always challenge Lu Ten to an Agni Kai, but whether he won or loss, Lu Ten would probably never speak to him again. Then he'd be stuck with just Zhao.
If being Firelord meant not having Lu Ten, Ozai would stick to just being a prince.
Soon enough, all of his scheming ideas stopped coming. An idea would rear its head, but he'd push it away. After a while, Ozai simply didn't care. He was a prince of the greatest nation ever; his life was good. Besides, his father looked like death warmed over. He could actually think of better things to do with his life than rule the world.
For instance, becoming the greatest Firebender in the world ever would give him glory, power, and he'd never have to go to any of those long, boring war meetings like his father.
He was such a genius. For a ten-year-old, at least.
Lu Ten elbowed him. "Why are you smiling?"
"I do that occasionally."
"Yeah. Right before you do something terrible to someone. It's not me, is it?"
Ozai grinned wider. "You? Lu Ten! You're my best friend. Why would I ever want to do something terrible to you?"
"It is me! What are you gonna do?"
Ozai gave him a smug laugh.
"Unless…" Lu Ten's eyes widened, realization and horror dawning. "You already did it! It was the cake, wasn't it?"
Ozai didn't say anything, and Lu Ten took his silence as confirmation.
"I knew that tasted weird. You monster!"
Lu Ten shot off, running to who knows where. Ozai peered out into the hall to see him collide with Lord Jinzo. The other boy clutched the man's robe and yelled,
"The cake is a lie!"
He ran off as quick as that, leaving the confused noble stunned. The man caught Ozai's look.
"Yes, that was the one who's going to be Firelord someday. Don't worry though. You'll be dead by then."