La, the Ocean Spirit, is one of two spirits which reside in the Spirit Oasis in the Northern Water Tribe's capital city at the North Pole. La and his counterpart, Tui, circle each other in an eternal dance of pushing and pulling.
Thousands of years before the Hundred Year War, La and Tui decided to manifest themselves into 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. La's physical form resembles a black koi fish with a large white spot on its head.
Centuries later, the Fire Nation initiated a siege on the Northern Water Tribe, during which AdmiralZhao captured and killed Tui, the Moon Spirit. Aang entered the Avatar State and walked into the water of the Spirit Oasis, with the Ocean Spirit swimming around him. The two fused to form Koizilla, a colossal fish-like figure. Together, they quickly annihilated the Fire Nation land forces and navy fleet with their superior waterbending. When La noticed that Tui had been revived by PrincessYue's sacrifice, the spirit separated from Aang, leaving him at the city's ice walls and returning to the Spirit Oasis. On his way, he encountered a dueling Zhao and Zuko; to avenge the attack on Tui, the Ocean Spirit formed an arm out of water and dragged Zhao below, never to resurface again.
Koizilla was the being that Aang became when he went into the Avatar State and fused with La, the Ocean Spirit. Admiral Zhao thought that in order to conquer the Northern Water Tribe, he would have to eliminate the moon by killing the mortal form of the Moon Spirit, Tui. He went to the Spirit Oasis and killed the white koi fish, which consequently darkened the moon and rendered waterbenders helpless. Furious, Aang entered the Avatar State, waded into the water, and merged with the Ocean Spirit, La, forming Koizilla. Made entirely of water and light, this destructive creature is literally the wrath of the ocean in physical form. Koizilla ventured throughout the Northern Water Tribe and wiped out dozens of Fire Nation soldiers and tanks before taking out Zhao's Fire Nation fleet. After seeing that Tui was alive again because of Yue's sacrifice, Koizilla separated back into Aang and La. As La retreated to the oasis, he grabbed Zhao, who was fighting Zuko, and pulled him under, never to be seen again.
"La" (拉) means "pull" in Mandarin Chinese, compared to "Tui" (推), which means "push". According to Princess Yue, the ancient waterbenders saw the tides push and pull, giving a hint to the Ocean and Moon Spirits' existence in the tribe.
The creators of Avatar nicknamed the fusion of La and Aang "Koizilla".
The Ocean Spirit's second form greatly resembles the Night-walker (Daidarabotchi) form of the Forest Spirit from Hayao Miyazaki's film "Princess Mononoke".
Tui and La resemble the Chinese yin-yang symbol, a symbol of balance and Taoism. Their coloring was especially indicative of this, as was their constant encircling of one another. As well, Aang pictured the two as a yin-yang symbol shortly before entering the Spirit World; this was specifically pointed out by Koh.
La represents Yin in appearance and Yang in personality on the yin-yang symbol. Yang is typically referred to as masculine and is associated with being active and strong while Yin is typically referred to as feminine and is associated with being cool, dark, and watery.
A petroglyph depicting Tui and La encircling each other was present on the stone wall behind the waterfall in Tarrlok's office.