The previous Chronicles of the Avatar volumes (The Rise of Kyoshi, The Shadow of Kyoshi, The Dawn of Yangchen, The Legacy of Yangchen) explored the stories of Avatar Kiyoshi and Avatar Yangchen.

The statue of Avatar Yangchen actually appears in the Eastern Air Temple in the Avatar Universe. In episode one, Gyatso breaks the Avatar news to Aang in front of a statue of Avatar Yangchen ...

The Avatar is an individual tasked with mastering ... like the oft-maligned Kuruk and the venerated Yangchen, as well as tidbits of information about geography, political history, and language ...