Just some key information:

Bold - the text
Italics - what the characters say in the present
Normal - flashbacks

Also, the dialog might not be accurate to the cannon story.
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"What's a soulmate?" the young prince of the Fire Nation asked his grandfather.

The old man thought about it while stroking his long white beard and then crouched down to the seven year old's level. "It's a.. Well, it's like a best friend, but more."

When Zuko first saw the young girl, who was his age too, he was intrigued. He wanted to know more about the (h/c) haired daughter of a Fire Nation nobleman.

He didn't know why the nobleman and his daughter had come to his home, but when Zuko's mother told him to go and play with the young girl, he immediantely took the chance to know her.

"Hey, I'm Zuko." the first born of Prince Ozai said happily.

"(Y/n). It's a pleasure to meet you Zuko." the young girl with (e/c) eyes said happily as well.

And just like that, these two started a friendship that would change their lives forever.

"It's the one person in the world that knows you better than anyone else."

Both of them were now ten years old. Both of them often hung out in the Royal Palace gardens. Just like they were now.

(Y/n) was weaving a flower crown quietly as Zuko was just absent mindedly looking at the pond where the turtle ducks were swimming in.

Finally (Y/n) sighed and spoke. "Tell me what's wrong."

Zuko was about to reply that nothing was, but before he spoke his friend cut him off. "And don't you dare say that nothing is wrong."

"How did you know?" Zuko asked.

"Zuko, I have known you for many years." she then tied the flower crown together and placed it on Zuko's head. "I've learned the way you act in certain situations. And right now, you're upset about something."

"It's about my uncle." Zuko started. (Y/n) nodded for him to continue. "His only son just passed away."

The girl didn't say anything. She just hugged the boy and he returned the gesture.

"It's someone who makes you a better person, well, actually," the old man said to his grandson."they don't make you a better person... you do that yourself-- because they inspire you."

Zuko and (Y/n) were now sixteen. The girl with the banished prince and his uncle to regain his honour. And right now they were staying in a cave while Zuko went to get supplies.

After a few hours Zuko had returned with much more supplies than expected. The girl looked at them wearily.

"Where did you get these?" (Y/n) asked Zuko in a whispered.

"Just found them somewhere." Zuko replied nonchalantly.

"You stole them, didn't you?!" The girl said angerly when she saw through his lie.

"So what if I did?! It doesn't matter anyway-" Zuko started yelling.

"Zuko-" Iroh was trying to calm him down.

"You can't just do that! It's wrong!" the (h/c)-ette yelled at her friend. "Someone might've needed more than we did. If we needed something we could've worked for it. Not-not steal from innocents!"

The words stunned slightly the ex-Prince. She was right. He knew she was.

Zuko sighed tiredly. "Well, it's too late now. I can't just go back and give it to them."

"A soulmate is someone who you carry with you forever."

It was late at night. Iroh was asleep in his room in the newly bought tea shop. (Y/n) and Zuko were on the rooftop, just enjoying each others company while watching the starry night. While the girl was enamored by the beauty of the night, Zuko's mind was plagued by a single question that he was meaning to ask the girl who was sitting becide him.

"(Y/n)." said girl's head turned to him and she hummed, stating that she was listening. "Why did you come with me?"

The (e/c) eyed female rose an eyebrow questiongly and Zuko continued. "You had a prosperous life back at the fire nation. You could've become someone great. So why did you want to follow me? A banished, disgraced prince-"

"Because a life without you" she said as she then turned to him fully and gently grabbed his hands. "wouldn't be prosperous for me if you weren't in it. Zuko....to me you are all that I need."

"It's the one person who knew you, and accepted you, and believed in you before anyone else did or when no one else would."

(Y/n) was in her room at the Fire nation Palace, reading a book. She wasn't satisfied by the choices Zuko had made recently, but she still forced herself to be happy for him.

But all too soon her peaceful reading time was inturrupted by a knock on the door.

"Come in!" She yelled, thinking that it was a maid or something.

The door opened and in stepped Zuko. (Y/n)'s eyes widened and she then patted the space on her bed becide her for him to sit. Zuko gladly obliged and looked at her.

"(Y/n)....there is something I need to tell you." Zuko said and said girl nodded.

Zuko sighed and grabbed her hands in his slightly larger ones and looked her in the eye. "I want to join the Avatar and help him defeat my father."

Silence.

"I know this is a bad idea bu-"

(Y/n) then suddenly hugged him tightly and soon enough he relaxed into her hold and hugged her back.

"Take me with you." she whispered in the boys ear.

"But aren't you mad that I'm leaving the-" Zuko asked in disbelief until (Y/n) cut him off.

"I understand why you want to do this." she said. "So take me with you. You don't deserve to do this alone."

" And no matter what happens..."

The war had ended many mounths before. Zuko took the throne of the Fire Nation along with the help of Team Avatar and (Y/n) became a master sailor. But right now, it seemed that luck wasn't on her side.

"Captain!" a crew member yelled over the poaring rain. "A huge wave is coming our way!"

And sure enough, when (Y/n) looked to where the man was pointing, she saw a large wave that would surely take the boat underwater. Along with it's crew members.

So she closed her eyes and accepted her fate.

"I love you Zuko." (Y/n) breathed out before the wave crashed down onto the ship.

"...you'll always love them."

The now retired Fire Lord finished his explanation. The young boy before him stared at him cureously.

"Was my grandmother your soulmate?"

"No......I lost my soulmate a long time ago. Even before I met my wife."

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I know, this wasn't the greatest oneshot. I'm just having a writter's block so I can't come up with new and good content.

Sorry about that.