La

"spirit"

- Our strength comes from the Spirit of the Moon, our life comes from the Spirit of the Ocean. They work together to keep balance.

La, the Ocean, is one of two spirits which resides at the Spirit Oasis at the North Pole, in the Northern Water Tribe's capital city.

History
Thousands of years ago, La and his partner Tui 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.

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 the Moon, 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, causing the spiritual equivalent of a lunar eclipse, destroying (or disabling) the power of the Waterbenders worldwide. To the horror of Katara, Aang, Sokka, Yue and General Iroh, Zhao kills Tui, destroying the power of 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 enters the Avatar State, fuses with La and defeats the Fire Nation invasion force and bends massive waves that wash all their ships away from the Northern Water Tribe. When La notices that Tui has been revived by Yue's sacrifice, it separates from Aang, leaving him at the city's ice walls, and returns to the Spirit Oasis, but not before taking Zhao, who was fighting Zuko. In a sense of poetic justice, Zuko attempts to save Zhao, but Zhao's pride keeps him from accepting. Zuko is the only person that knows Zhao's fate.

Koizilla
Koizilla was the being that Aang became when he went into the Avatar State and fused with La, the Ocean Spirit. Admiral Zhao went to conquer The Northern Water Tribe. Zhao thought that, in order to win, he would have to eliminate the Moon by killing the mortal form of the Moon Spirit, Tui. So he went to the Spirit Oasis and then killed the white koi fish which consequently darkened the moon and rendered the Waterbenders helpless. Furious, Aang goes into the Avatar State, then went into the water and merged with the Ocean Spirit La, forming Koizilla. Koizilla ventured throughout the Northern Water Tribe and managed to wipe out dozens of Fire Nation soldiers and tanks. Koizilla then took out Zhao's Fire Nation fleet. After seeing that Tui was alive again, thanks to Yue's sacrifice, Koizilla went back to being Aang and La. Then La grabbed Zhao, while he was fighting Zuko, seizing the Admiral. Zhao's ego made him refuse Zuko's help and ended his life.

Trivia

 * "La" (拉, lā) means "pull" in Chinese. The character is also used in Katara's name.
 * The creators of Avatar nicknamed the fusion of La and Aang "Koizilla".
 * The Ocean Spirit's second form greatly resembles the Night-walker (Daidarabocchi) Forest Spirit from Hayao Miyazaki's "Princess Mononoke".
 * Interestingly, Tui and La resemble the Chinese yin-yang symbol, a known symbol of balance and Taoism. Their coloring is especially indicative of this, as is their constant encircling of one another. As well, Aang pictures the two as a Yin-Yang symbol shortly before entering the Spirit World; this is specifically pointed out by Koh.