Tui, also known by her alias The Moon Spirit, is a minor protagonist in Avatar: The Last Airbender. She and La have made their first appearance in the episode "The Siege of the North".
Tui manifests as a white Koi fish that has a black spot. She is basically a colour-negative version of La. After Yue fused her life force with her as a reward she proposed, Tui's form warped into a humanoid form. In this form, she greatly resembles Yue, but with a translucent look.
Thousands of years ago, Tui, and her companion La, the Ocean Spirit entered the mortal realm to aid humans. They took the form of Koi Fish and took residence in the Spirit Oases, where the Northern Water Tribe would eventually be built.
When the North Pole was eventually settled, mortals learned how to waterbend by observing the moon and how it moved the waves, and came to worship Tui and La. From this relationship, was born the Northern Water Tribe.
Thousands of years later, Chief Arnook's newborn daughter Yue was stricken with a terminal illness and none of the tribal healers could help her. Desperate for a miracle, he brought Yue to the Spirit Oases and Implored Tui and La to save her. Tui responded by granted the infant a portion of her lifeforce, turning Yue's hair white. Though Arnook was overjoyed that his daughter was saved, he feared that one day Yue would have to repay Tui.
Fifteen years later, Aang and his friends arrived at the Northern Tribe to learn Waterbending, and were unknowingly bringing the navy of the Fire Nation with them, whom the Tribe was at war with. The fleet's leader, Admiral Zhao had learned of Tui while in Wan Shi Tong's library and discovered that if he killed her, all waterbenders would lose their power. During the following battle, Zhao and his fellow commander General Iroh broke into the Oases and captured Tui. In response, Tui turned the moon red as a warning to Zhao. Iroh warned Zhao that Tui's death would throw the world out of balance and threaten even the Fire Nation. But Zhao refused and slew Tui. The moon dissapeared and Waterbenders lost their powers.
Enraged, La merged with Aang and annihilated the Fire Navy and most of the remaining soldiers. Yue however realized that since a piece of Tui's life force was inside of her, she must return it in order to resurrect Tui, but such an act would kill her. Despite Sokka's protests, Yue gave her lifeforce Tui. With her lifeforce restored Tui was resurrected. When La found out his companion had returned to life, he and Aang separated.
In addition, with Yue's lifeforce bonded with her own, Tui and Yue merged together.
- Including Roku's death in "The Avatar and the Fire Lord" and Aang's fatal injury sustained in "The Crossroads of Destiny", from which the latter was resurrected shortly thereafter, Tui was the first of three our of eight characters to have been observed dying or being murdered in the original series. In chronological order, the others were Yue, Chin, Jet, Aang, Fang, and Combustion Man. But like Aang, Tui was resurrected shortly after.