A/N: So this an AU where Zuko starts spying for the "rebellion" almost right after that war meeting where Ozai decided to turn the Earth Kingdom into a giant burnt crisp, because Zuko realises this isn't what he came home for, so he fights with them on the day of the eclipse.
Disclaimer: I don't own A:TlA, all rights go to two middle-aged white guys and a big company.

Chief Hakoda cleared his throat, calling all the warriors gathered on the small, rocky island before the Gates of Azulon to attention.
"Alright, everybody, our spy is supposed to meet us here, so let's just hope he," he received a pointed look from his daughter, "or she shows up soon."

"I don't need to show up, I'm already here" a husky voice told them from above. All looked up to its supposed origins.
There, on a bigger pile of rocks, was none other than the former(?) ex-prince of the Fire Nation. Zuko. Katara growled something unintelligible next to Sokka, who was making strange noises of incomprehension, and Toph and Aang, who were grinning broadly.
Most of the allied warriors looked insanely confused, some smiled welcomingly, and some looked worried.

Zuko made his way down the pile of rocks until he was standing right in front of Katara.

"Katara," he started, giving her an unsure look. The waterbender rolled her eyes, but let him continue, "I'm sorry for Ba Sing Se. I messed up."

"You could say that," she muttered. Zuko pinched his nose and looked her firmly in the eye.

"And I would like you to know that you were a big part of why I started spying. What you said in Ba Sing Se...All of it awakened something inside of me. It took a while, but it did and I am eternally grateful, but I know you probably don't trust or believe me right now, so I promise that I will do everything in my power to make Ba Sing Se up to you."

Several mutters of "What happened in Ba Sing Se?", answered by "I don't know" echoed through the crowd, but everybody's gaze was transfixed on the two teens.

Katara gave Zuko a hard look.

"I hear your apology. I sadly cannot give you a firm answer, so I have decided to trust you as an ally for the assault. After that, we'll see."

-oOo-

They saw.

They saw them fall in love on that very battlefield. The sparks were subtle and unseen by all but a few, but they were enough to make their love burn.