Avatar Roku was the previous Avatar incarnate, and is thus an aspect of Aang's self. Born into the Fire Nation in life, Roku is a benign force in spirit, serving as Aang's advisor and protector throughout the series. During the episode "Avatar Roku ", Aang was able to communicate with Roku by visiting the Fire Temple that Roku called home 112 years ago. There Roku informed Aang of the impending return of Sozin's comet. He also described Fire Lord Ozai and the Fire Nation's ability to harness the comet's energy to finally win the war. Roku also manifested himself with Aang to defeat a group of Firebenders holding his friends hostage. He has guided Aang several times since, appearing to Jeong Jeong to persuade him to teach Aang Firebending, offering critical wisdom in the ways of the Spirit world during the great siege of the North Pole in order to defeat the invading Fire Navy and save the Moon Spirit, and most recently, revealing to Aang the nature of his Avatar State. He can thus be seen as a kind of mentor or spirit guide for the current Avatar, Aang; this may be because Aang has no appointed teacher (the Air monks having been killed), or it may be possible that all Avatars have such a guide during their lives.

Roku possesses a spectral animal guide, a dragon that served him in life, in the same capacity as Appa does to Aang.

Avatar Roku first appears as a statue within a shrine in the episode "The Southern Air Temple," and Aang mentions that although he never met him, he felt a particular connection to his previous incarnation . In the episode "Avatar Roku ", Aang finally meets the spirit of Roku for the first time.

Avatar Roku is the Avatar seen to demonstrate mastery of all four elements in the opening animation of each episode.