La is a minor protagonist from the Avatar series. He and his companion Tui made their debut appearance in The Siege of the North.
Just like his female counterpart Tui, La takes his basic form's design from a Koi fish. La's appearance, however, depicts him as a photonegative version of Tui. His second form is basically a bigger, bluer, anthropomorphic version of a Koi fish, and is attained when Aang and La fuse their life forces together to avenge Tui after Commander Zhao destroys her.
In the past, Tui and La crossed the barrier between the Spirit World and the Mortal Realm to give waterbenders the ability to bend water. They made a new home in the alcoves of Agna Qel'a, creating the Spirit Oasis that would later become not just their home, but also the Northern Water Tribe's greatest landmark.
After he learned of their existence, Commander Zhao decided to disrupt the balance by capturing Tui, who was necessary for waterbending. Even though his retired general, Iroh, warned him that killing Tui would cause chaos even for his people, Zhao, now promoted to Admiral, decided to destroy the Moon Spirit anyway. Because he had lost his sister, La swam around frantically in the pool.
Acknowledging the situation, Aang realized that he and La could work together to save the world. Aang then fused his life force with him, transforming La into a giant humanoid Koi fish that proceeded to decimate the Fire Nation. La then proceeds to trap Admiral Zhao in the Spirit World for his crimes. Zuko, despite his rivalry with Zhao, tried to help him, but failed, leaving Commander Zhao to spend his life in the Spirit World.
La later finds that Yue had given up her life force so she could revive Tui as a reward for helping her, as Master Pakku had foreseen in a dream. He returns home, releases Aang, and revisits his companion, who helps him restore balance to the world just like they always did.