Disclaimer: I don't own Avatar: The last Airbender. Or Yue.
Three Second Sunrise
Luna
I had to leave them all behind. Father, Mother. Sokka. I'll admit it; I was upset about it at first. The fact that I had no other choice didn't change that. But that faded with time. I realized that I couldn't let the world fall because of my selfishness.
I didn't know what to expect with becoming the moon spirit. Just existing maintained the balance of the world, so that was less stress there.
The koi in the oasis at the North Pole weren't actually the spirits them- ourselves. They were more like an anchor for us in the physical world. In a sense, they were our hearts. We could go anywhere in the physical world we wanted, but if the fish were injured, we would be too.
Tui was kind enough to me, although a bit distant. That's understandable. He was a spirit, and I was just a human until La was killed. He considers me a child, as do the other spirits. Compared to them, I am just an infant.
I kept watch over Sokka a lot. I didn't want him to be upset about my fate. I had not choice, and he realized that. He was still sad, though. He never admitted it to Aang and his sister, but he was.
Some places in the physical world are closer to the spirit world than others. In those places, I can appear to the living whenever I want, although the phases of the moon dictate my strength. When Sokka was in Foggy Swamp, I appeared to him. I tried to tell him not to worry, that he didn't have to protect me, but the moon was new and I was weak in the physical world. He misunderstood me.
Later I was able to correct my mistake. Sokka had run into another girl, presumably an old friend. She liked him. He liked her, but was afraid to admit it.
The moon was full then, so I could appear to him and be understood completely. I explained that he didn't have to protect me at the North Pole, and that all I wanted was for him to be happy. There were no misunderstandings.
Suki, her name is. They make a good couple.