Raava

"Vaatu cannot win. Do not give in to ten thousand years of darkness. You are the Avatar."

- Raava encouraging Korra to keep on fighting.

Raava is the spirit of peace and light and, after fusing with Wan, the Avatar Spirit. She is one of the oldest known spirits, having existed over ten thousand lifetimes before the appearance of the first humans. She and Vaatu, the spirit of darkness and chaos, spent much of their existence combating each other, at least since the Harmonic Convergence in 19,829 BG, with neither able to fully vanquish the other. Even if Raava were to destroy Vaatu and usher in a new era of peace, what little darkness existed in Raava would magnify until Vaatu was reborn and burst forth, beginning the cycle again. The reverse would happen to Vaatu if he were victorious over Raava.

During the Harmonic Convergence in 9,829 BG, Raava permanently merged her essence with Wan's, creating the Avatar. After her demise in 171 AG, Raava lost her connection to Wan and all the Avatars after him, but was able to merge again with Korra, recreating the Avatar and starting the Avatar Cycle anew.

Trivia
In some ways, Vaatu share some similarities with Anu from Diablo series, where both are light beings with similar symbolism(Light and Peace), and trapped with a battle with their dark equivalent(Vaatu for Raava and Tathamet for Anu). Unlike Anu and Tathamet, however, Raava and Vaatu are separate entities, while Anu and Tahthamet used to be one single being due to Tathtamet's true nature was embodiment of all negative inside Anu.