Zuko was in love with Katara. By this point, he'd resigned himself to the fact that he probably always would be. The war had been over for two years, it was two years since he'd resumed his relationship with Mai and Katara had accepted Aang's feelings and yet, after all that time, his love for her only grew stronger. Sometimes he felt bad about leading Mai on, but then she'd say something rude or dismiss him and he was reminded that she didn't care about him either. For both of them, it was solely a political relationship, one where he would have a Fire Nation nobleman's daughter on the throne beside him and she would get her "perks".

That's all she actually cared about, those so-called "perks". Perks are supposed to be added bonuses, nice things that sweeten the deal, but for her, those and the throne were the deal. He'd given up on trying to have real conversations with her long ago. The one or two times he tried to talk to her about his mixed feelings regarding his new position as Fire Lord, she waved him off and told him to stop being dramatic. Right, because suddenly being in charge of running a country and rebuilding said country and the entire world isn't actually something to be concerned about.

What hurt the most was how happy Katara appeared to be with Aang. He'd been skeptical when they first got together, mainly because of how young Aang was, but, apparently, his skepticism was unfounded. As such, whenever they visited, he always challenged himself to appear equally happy with Mai. The last thing he wanted was for Katara to see that he was unhappy in his relationship while she was perfectly happy in hers or, even worse, realize how he felt about her while she clearly didn't feel remotely the same. By this point, he had perfected the skill, and it was a good thing, too, because she and Aang would be arriving momentarily.

...

Katara was nervous. It had been months since her last stay in the Fire Nation and, after that experience, she didn't know if she could do it this time. She hated to admit it, but she was in love with Zuko and had been for a while. It was selfish, and she felt awful about it, but the only reason she said yes to Aang was because he'd gone off with Mai. She had thought that it would get better with time, but it only continued to worsen. She'd been with Aang for two years and he'd been with Mai for the same amount of time, so she couldn't understand why that was the case. Two years should have been enough time to move on.

She felt terrible about leading Aang on, she really did. However, when he'd ditch her for his fan club or when he'd completely fail to even try to understand her, the guilt lessened. Katara knew that he really liked her, of course he did, but she also knew he didn't actually love her, even if he claimed to. Over the years she'd noticed that he was only ever interested in her when she looked good or when she was willing to put important stuff aside and have fun with him. He never wanted to talk about important things and always resented when she would hold him to his responsibilities. He loved her the way he loved Momo- a cute thing that he can play with.

The worst part was how genuinely happy Zuko was with Mai. She'd been apprehensive about their relationship, based on the times she'd encountered Mai, but she had been proven wrong. Whenever they visited, Katara was always mindful to make sure her mask was seamless and that she played the role of Aang's happy girlfriend flawlessly. She didn't want him to see that she was discontented with her relationship when he was clearly so happy with his, or, worst of all, for him to realize how she felt. As their ship approached the harbour, Katara slipped on her mask, blending the edges flawlessly.