The day she was born her mother knew she was different, but not in the least how much.
Fast forward to the day the counterattack against the fire nation gravely failed. Aang and his squad had to leave, unknowing that the prince was right behind them, following them. They were on their way to the air temple, hoping it would be the safest place to be.
The air temple was right beneath a cliff, before a huge canyon.
They made themselves comfortable. Aang wanted to strive around a little with the other kids but Sokka held him back, reminding him they had to first think of another plan to defeat the fire lord before the comet arrives.
Just as they agreed to stick to their previous plan, containing that Aang has to fulfill his destiny as the avatar and master all the four elements within the little time that was left, someone appeared out of nowhere.
A girl, probably around three to four years older than them. She was extremely skinny and dragged her long blonde hair behind her. She ran away as soon as she saw them.
"Quiet an unusual color", Katara thought.
"I can't feel her anymore!", Toph cried out and right when Aang was about to pursue her (thinking she is probably a ghost), another person appeared in their field of vision.
"ZUKO!" they shouted in unison while getting ready to defend themselves. He greeted them, trying to apologize and offering to either become the teacher of the avatar, or his prisoner. But since team avatar and the prince share a painful past together, each of them in their own way couldn't believe even one word he said.
After he left, only Toph urged the others to rethink his offer. Since Aang hasn't much time left and the possibility to find a teacher for him is extremely low, this would save them a lot of time and effort.
Naturally the others couldn't ignore their hatred and disagreed so Toph stomped off, leaving them behind.
Unfortunately when she approached Zuko's camp, he instinctively used his fire to defend himself (thinking there was an enemy) and burned her feet. Returning to the camp the other's opinion of Zuko got even worse and Toph already believed they would never find a teacher for Aang.
After a number of events they fortunately changed their opinion (even just a little) and let him join.
They already forgotten about the ghost girl from before.
Then, after Zuko and Sokka returned from their little "fishing trip", bringing back some new friends, she appeared again.
This time it was Sukki who saw her first. She asked: "Who is that girl over there?" and as everybody turned around, instantly Aang started running towards her. Even as she heard him and turned around he was sure he would capture her, until she started running at the same speed as him. But he didn't wish to lose her so he sped up as much as he could.
In the end she tripped over her long hair, and he was able to catch up. As soon as he arrived next to her, he realized that she wasn't what he thought she would be.
Two mesmerizing amber colored eyes were glaring angrily at him, while he tried to help her up."You're an air bender, ...like me!". As everyone else arrived, the strange girl was already up again and was now glaring at each one of them until her eyes met those of a certain fire prince.
Suddenly she gave him the most beautiful smile he ever saw and jumped into a fierce hug. Now everyone was glaring at Zuko, expecting an explanation or hearing an excuse as to why she was so fond of him. But he was just perplexed and didn't know what to do. Toff noticed and snapped at the others to back off.
The girl finally let go of him and started talking: "Hello, my name is Aida. Welcome to my home". At this point no one knew what was going on as she continued: "It's been a long time since you visited me here, I believe the last time you came here you were with your uncle".
Zuko remembered coming here right after being banished from the fire nation, but couldn't remember ever seeing a girl here.
As he was about to explain to the others, Aang intervened: "Your an air bender … how is that possible? I thought I was the last living air bender".
As she grimaced at him, Zuko told her that those are his friends, and she shouldn't be afraid of them. So she answered: " Air bender ?... I don't know what that is but I doubt I am one of those", "But you must be, there is no other way you could run that fast or disappear into thin air while being 100% human!".
She started grimacing again, when they heard a scream. "We need to leave!" she said calmly but with a worried expression. "Please...take me with you".
To Aang that was the perfect opportunity to find out if she really is an air bender or not, so he agreed without even consulting the others. They left as soon as they could and while looking behind Sokka and Katara couldn't hold it anymore and asked what it is their running from, only to receive a one word answer that left them even more confused than before. "Mother".