Tui

'Tui, the Moon Spirit (and the Moon itself), was on of two spirits which resided at the Spirit Oasis at the North Pole, in the Northern Water Tribe's capital city.

History
Thousands of years ago, Tui and its counterpart La decided to manifest themselves in the physical world as to benefit mankind, taking up residence in an alcove of the Northern Water Tribe's capital city. Thus, the Spirit Oasis in the city became its most treasured landmark and most defended point.

When she was a baby, Chief Arnook and his wife took their baby Yue to the Spirit Oasis because she was dying. Although Tui did grant healing to Yue, it prophesized that one day Yue would have to take its place as Moon Spirit. Chief Arnook received this prophecy in a dream; although he resented it, he accepted it as truth.

Tui had been correct - many years later, the Fire Nation launched a siege on the capital city of the North. Steadily, the Fire Nation navy advanced upon the city, eventually destroying its protective locks and dams and advancing onto its white marble surface. Admiral Zhao, the leader of the attack, advanced with his personal guard to the back of the royal palace to kill Tui, as he had thought to do so ever since he had discovered the existence of the Spirits in the mortal world from Wan Shi Tong's library. In a scoop, Zhao caught Tui in a bag of water, and a lunar eclipse occurred, destroying the power of the Waterbenders worldwide. To the horror of Katara, Aang, and General Iroh, Zhao then kills Tui, destroying the moon altogether. Iroh, in anger, lashes out at Zhao, incapacitating his guards. Zhao, however, slips away, only to be attacked by Zuko. Meanwhile, Aang fuses with La and destroys the Fire Nation invasion force.

Although Tui is dead, Yue later sacrifices herself to it, becoming one with the Moon Spirit.