Three
At first, they were three.
Well, really they had been two and one, in Aang's own humble opinion, but Katara had insisted that Zuko had proven himself worthy, and they were three. Katara could have said that there were no Air benders left, and Aang would have believed her. So they were three; three fighters, three travelers, three best friends. Sokka and Toph were there of course, but to Aang, they had always been two, lost in their own world of, whatever there was going on between them.
One did not intrude on the world of two; it was an unspoken rule, something established between the two and the rest, maybe even without Toph and Sokka's knowledge. But it was there, and respected, just as Toph and Sokka did not intrude upon the world of three, that was that.
At night, the three of them sat and talked, told jokes and old stories, shared secrets and discussed their latest adventures in depth, laughing shakily at their close escapes. It was always the same; the three lounged together, usually Zuko sitting on a rock or a log, in the middle of the two, chin in his hands, and golden eyes reflecting the light of the fire. Aang would sit across from him, soaking in the peaceful feelings that radiated from his friends, and realized he had never really felt as home as he did when they were three. Katara sat in the middle of the two, sometimes stretched out on her bedroll, other times standing and talking, animated with her hands and whole body, telling stories of her Gran-Gran and her tribe's tales. They were three, and they were happy.
But then, things changed. Somehow, Katara always ended up nearer to Zuko in their circle. It started as a simple lean of wait in the general direction of Zuko, propped on one arm, head tilted to the side to fix Aang with her blue gaze, but her body was only aware of Zuko. When he shifted, so did she.
They were small things really, small touches on the arm, or a ruffle of the hair, and a smile at random, or something said to the general three of them, but seemed to be directed solely for only one person. They were small, but they were there. One night, Katara had stretched her legs out, and Zuko, seemingly unthinkingly, lifted them into his lap, tracing his fingers over her dark ankles even as he told Aang of a finer point in Fire Bending that the young Avatar hadn't known.
The three had become two and one, overnight it seemed. But when Aang looked at their past relationship, maybe, he thought, they had always just been two and one, just not in the order he had thought.