Tui, the Moon Spirit, is one of two spirits that reside in the Spirit Oasis at the North Pole, just outside the Northern Water Tribe's capital city. Tui and her counterpart, La, circle each other in an eternal dance of pushing and pulling. Tui is of the few entities known to have traveled into outer space.
Thousands of years before the Hundred Year War, Tui and La decided to manifest themselves in the physical world to benefit mankind, taking up residence in an alcove of the Northern Water Tribe's capital city. They created the oasis in the North Pole and took the form of two koi fish. Thus, the Spirit Oasis in the city became its most treasured landmark. Tui took the form of a white koi fish with a large black spot on its head.
When PrincessYue was a baby, she was taken by her father, ChiefArnook, and his wife to the Spirit Oasis. She had been born gravely ill, very weak and on the brink of death. He dipped her in the pool at the Spirit Oasis and prayed to the spirits to save her life. Immediately, Yue's black hair turned white and she began to cry. Although Tui had healed Yue, Chief Arnook had a dream that one day she would have to take Tui's place as the Moon Spirit.
Many years later, AdmiralZhao, on behalf of the Fire Nation, launched an attack on the capital city of the Northern Water Tribe. The Fire Nation Navy arrived at the city, eventually destroying its protective locks and dams and steadily advancing into its interior. Zhao advanced with his personal guard to the back of the royal palace to kill the Moon Spirit, as he had planned to do ever since he had discovered its physical existence at Wan Shi Tong's Library. Zhao caught Tui in a bag, causing the spiritual equivalent of a lunar eclipse and removing the power of all waterbenders worldwide. However, Iroh stepped in and threatened Zhao, telling him that whatever he did to the Moon Spirit, he would unleash on him tenfold. To the horror of Katara, Aang, and Iroh, Zhao abruptly murdered Tui after appearing to release it, destroying the moon altogether. In anger, Iroh lashed out at Zhao and incapacitated his guards. Zhao managed to escape Iroh, only to be attacked by Zuko. Meanwhile, Aang entered the Avatar State and became one entity with La; he defeated the Fire Nation invasion force on land. He took care of the fleet by bending massive waves that washed all their ships away from the Northern Water Tribe.
Iroh realized that Yue held Tui's life energy inside of her, since it had given some of its life to her when she was born. Yue made the choice to sacrifice her life in order to resurrect Tui over Sokka's objections, since he was tasked with protecting her. After she brought back Tui, not only did the moon return to its full power, but Yue's physical body disappeared from Sokka's arms and reformed as Yue's spirit form, making her the new Moon Spirit. In this form, she kissed Sokka one last time and disappeared.
By having her life force re-bound with Tui, Yue not only became Tui's successor, but also gained the ability to waterbend, a trait she had not carried in life. She demonstrated her abilities to some extent when she generated a massive wave on which Aang could ride toward the island where Roku's temple once stood.
"Tui" (推) means "push" in Mandarin Chinese, compared to "La" (拉), which means "pull". According to Princess Yue, the ancient waterbenders saw the tides push and pull, giving a hint to the two spirits' existence in the tribe.
In The Last Airbender film, Zhao associated the Moon Spirit with Yang (white, male) and Pull (拉, Lā), a different convention than the animated series. As such, Yue referred the Moon Spirit as male, "He gave me life when I was a child."
It is ironic that Zhao killed the white, masculine counterpart of the Yin and Yang symbol as in Taoism, it represents the sun and fire, while Yin, the dark and feminine part, represents the moon and water.
Including Aang's fatal injury sustained in "The Crossroads of Destiny", from which he was resurrected shortly thereafter, Tui was the first of nine characters to have been observed dying or being murdered in the original series, though she was resurrected soon afterwards by Yue's sacrifice. In chronological order, the others were Yue, Zhao, Chin, Jet, Aang, Roku, Fang, and Combustion Man.
The Moon is considered the original waterbender. Members of the Water Tribe used to watch how the Moon pushed and pulled the tides and learned how to do it themselves, which makes waterbending the only bending art that was not learned from an animal species.
A petroglyph depicting Tui and La encircling each other was present on the stone wall behind the waterfall in Tarrlok's office.