Before the failed invasion, Aang could remember having nightmares about fire consuming Katara. He mentally told himself—he would not let that happen.
He failed.
Not in the literal sense, because Katara was sitting across from him, happily chattering to Suki about something or the other. Aang wasn't really paying attention. Every few minutes or so, the Water Tribe girl that he had grown so attached to would glance to her left where a certain Fire Lord was standing off to the side talking to Sokka. Normally Aang wouldn't have given the looks so much thought. But see, occasionally Zuko would look over his shoulder and peek at Katara.
Therein was the problem.
It continued, even when Katara cupped his cheek, held his hand, or gave him a chaste kiss on the lips. Aang was staying in the Fire Nation in order to support Zuko in his first few years as Fire Lord and Katara had devoted herself to being wherever Aang was. He happily kept her there by his side because he wanted her there.
But then the looks turned into smiles. It started at mealtimes during their first few weeks there. Katara would look up from her plate and see Zuko frowning down at his. But those upturned lips and kind eyes always made Zuko break out into his own cheerful expression. Aang didn't know how to react. He thought those smiles…weren't they meant for him?
Eventually the smiles turned into touches. When the Fire Lord looked upset, Katara would hug him. When the waterbender was fretting over something, Zuko would give her hand a gentle squeeze in support.
Eventually the touches turned into conversations. It was only a matter of time before the two of them developed a bond. They understood each other in ways that were beyond Aang's grasp. His twelve year old mind—wise beyond his years but still so like a child—could only sum it up to the fact that sometimes Zuko was sad, and that even though she smiled a lot, sometimes Katara was sad too. He wondered how anything besides happiness could bring two people together, but even the Avatar did not understand the true depths of human feeling.
Aang was starting to get anxious. Zuko was beginning to bring Katara up in everyday conversation, even if she was completely off topic. Then he would trail off, lost in his thoughts before turning his attention back to the matter at hand. Zuko seemed happier now, more confident and determined. Mai had stopped coming around.
Katara seemed the opposite. She looked shaken, sometimes scared. And she was starting to watch what she was saying around the young Avatar, something that had never happened before. It didn't occur to Aang that she was afraid of him. Of what his reaction to the whole ordeal might be.
He thought back to those nightmares. He wondered if a person could consume another human being. Because it looked to him that, despite his efforts, Katara had been consumed by fire. Consumed by Zuko. Aang thought briefly about leaving the Fire Nation and asking Katara to come with him. She probably would, because that was who she was—loyal, even if it killed her.
But how far could one person go to appease their own selfishness? So Aang stayed. He decided to talk to Zuko.
"Do you love her?"
The Fire Lord balked, surprised by the unexpected question. "I…what—"
"You do. You love her." He wasn't angry. He wasn't even sad. He just wanted the truth.
Zuko was shaking. He looked terrified. But after a moment he nodded. "Yeah. I do." Serenity seemed to cross his face. He took a deep breath and repeated himself. "Yeah. I do. I love her. I love Katara."
Something stirred inside Aang, an emotion that did not belong to him, the twelve year old airbender, but to the thousands of Avatars who had come before him, and who inside of him made one living unity. One living soul. It was a freeing sense of release, of happiness. Happiness for Zuko, for Katara. Love was extraordinary and so difficult to find, despite the stories. Who was he to frown upon that?
Zuko sat down and Aang joined him, listening to the Fire Lord explain himself. It was a surreal experience. Aang felt the beginnings of a smile. Yes, Katara had been consumed by fire, but Zuko had been drowned in water.