I'm going to write this in conjunction to my other story
Prologue
10 years earlier
Roku frowned. He was bored. He was in front of a large body of water in the Spirit World.
Getting up, the former spirit mentor of Avatar Aang closed his eyes and focused. He guessed he could go out into the world but there was nothing to do.
He detected a tinge of energy and stopped, he was scouring outside the boundaries of the spirit world and investigating what looked to other worlds without the avatar.
Alternate dimensions, interesting. He looked down into the water and saw a young boy around five rush into to help someone only to be thrown away and beaten up. He felt anger as the men raped the woman and walked away.
Horrible, Roku thought. Can I help him?
He paused then smiled.
He most definitely could, that world had no avatar it had those two heroes Batman and Superman but otherwise nothing else. The boy was smart, intelligent and would be a good fighter one day but he needed to be on the right path.
Roku then began gathering his energy for something he didn't even think was possible. To successfully do this, he may need Kyoshi's help for martial arts training which he rarely asked for.
…
Ajax Powers shot awake as he felt something inside of him. He looked around, the area was empty, no-one was around and the woman had been hurt. To his surprise his wounds were healed and his clothes had been mended.
He felt a tinge of anger and he balled his fists. He couldn't be like his father, why? His parents had been killed whilst he was young, leaving him defenceless and homeless. He sighed. New York City really was a bad place.
"Hello boy."
Ajax shot up. What was that? There was no-one around right?
"I'm in your head."
"Oh."
"I saw what you did earlier, it was very brave."
"I failed."
"Yes, you did. But I just gave you power to help the defenceless. To restore balance to the world."
Ajax grinned then stopped. "Look, mister if you're trying to get me to be some sort of …"
"Let me show you boy!"
Then Ajax saw it, he saw the start of the avatar world and the beginning of the avatar he saw the four bending arts, he saw Roku and Sozin, he saw Roku mastering the bending arts and Avatar State, he saw the war starting and the war ending, Aang dying, Avatar Korra stopping Vaatu and he saw what Roku had done. He then saw the powers that he had along with the four bending arts.
He gasped. "You were a hero."
"Yes. The cycle of the Avatar is endless and keeps going. But I realised that I was not needed on my world anymore, so I came here to make a difference in this world."
"I'm only five."
"I know. You will just be practicing your powers for now until otherwise."
Ajax paused. He really had nothing else to do, his parents were dead and he was being offered a chance to be a protector like his father had been. He could help people, save them! Be a hero!
"Fine." He got up and began to walk. Then, "Seriously spike bombs? Is that the best?"
"A side effect of transferring my spirit into your body. You have your own Avatar State with you combining Kyoshi and mine's spirits."
"You didn't answer the question."
"Shut up and find somewhere to train!"