I. Mai
The first time Zuko saw his childhood friend as a girl, not just a friend but as a girl, was when he first began returning her shy smiles and talking to her more than once a week. Then it seemed that in a single moment he found out that she returned his feelings.
Their relationship was an innocent, filled with walks next to the pond and the hesitant kisses that seemed to make them seem younger than ever. However, Zuko could truly feel proud when he was the only one who could get her to smile.
But it seemed all at once the Agni Kai changed his life forever, and Mai was forgotten with the tide, as were the turtle ducks they had so fondly tended to.
II. Katara
Zuko had not paid much attention to girls during his banishment, but he couldn't help noticing the warmth her smile spread through him, the way her blue eyes sparkled in the sunshine. There were even a few times he found himself idly wondering what it would have been like if he were the avatar, but the thought provoked such a sudden rush of thoughts and emotions that he abandoned it immediately.
So Katara was washed away from his memory little by little, until it seemed impossible that he, the Fire Nation, could ever have had feelings, however miniscule, for such a worthless little peasant.
III. Song
There was something about her that was different than any girl Zuko had ever known; her sweet manner and kindness made him feel guilty about the things he'd done, the person he was. But it seemed all his doubts slipped away when she opened up to him, showed him that he was not alone in his pain.
But he couldn't stand knowing that the hurt they felt was completely different – he was hurt by his own father, yet still loyal to the nation that had banished him; she had been scarred by invaders that seemed to be inhuman.
Thus Zuko felt Song must pay for the guilt, but surely stealing her horse made him feel only worse. No longer could he think of her with out sharp stabs of pain spiking into his heart, so Song was pushed to the back of his mind with all the other memories too sore subjects to bring up.
IV. Jin
It was true, Zuko had first thought she was some sort of spy, but when she asked him to do something and gave him one of her looks, it was magic. It seemed he that she had come sort of control over him, a control he was never quite able to understand or acknowledge. Then came the Firelight Fountain and the short, tainted kiss, full of lies that were built on Li and truths that came along with Zuko.
He could not stand to be something he wasn't in such extremities, and what he had told her was indeed the truth: It's complicated.
V. Toph
Zuko began to notice her a year or so after the war, when she began to look less like a tough sort of boy-girl, and more like a young woman. Certainly their personalities meshed together well, but it seemed that there were too many fights, too many unsolved problems, for it too last. Certainly he loved her, but as to whether he was willing to spend the rest of his life with her was a totally different matter.
Although Toph understood him, Zuko eventually became aware that she scorned him, for groveling to his father as a child and not breaking away from the Fire Nation sooner. As soon as he realized how right she was, the sand their relationship was built on seemed to drain away all at once, until Zuko and Toph were no more.
VI. Mai
Zuko met her again, after the war, pleased and slightly shocked that she had become part of a group of Fire Nation rebels, giving them information about Azula and battle plans. He was even more surprised when Mai revealed she had decided to help when word came that he had become the avatar's Firebending master.
Once again he was captivated with her, with the smiles that were rare but more precious then anything else; with the careful accuracy with which she was able to do everything. Suddenly, it was just like old times, but with a different boy (stronger, wiser, freer) and a different girl as well. Zuko felt himself fall in love with her regalty, precision, intelligence, and everything came together on the night he proposed.
After all, Mai was like a girl Zuko used to know, once upon a time in the fairy tale land that had never been theirs in the first place.
Not sure if I got the ending right...
Review, guys!