Book 3 of Avatar: The Last Airbender reveals that Aang and Zuko are both related to the former Avatar Roku, but in a slightly confusing way.

Avatar: The Last Airbender shows Aang and Zuko gradually becoming friends, but they also share a common ancestor who lived before the Hundred Year War: Roku. As the titular last surviving airbender and the banished crown prince of the Fire Nation aggressors who decimated the Air Nomads, Aang and Zuko begin their journeys at very different stations in the world. However, they are both capable benders whose destinies bring them into contact with some of the world's most powerful forces.

From the beginning of Avatar: The Last Airbender, Aang and Zuko are pitted against one another, largely because the latter has been tasked with capturing the former in order to restore his honor in the Fire Nation. Circumstances and personal growth eventually conspire to bring the two together as allies in season 3, but their kinship began long before then.

The Last Airbender season 3 features an episode titled "The Avatar and the Fire Lord" in which, among other things, it is revealed that Zuko is the grandson of Avatar Roku by Roku's daughter, Ursa. After Zuko's other grandfather, Firelord Sozin, is complicit in Roku's death, the Avatar spirit passes to a newborn Aang. Aang is then eventually frozen for 100 years, awakening when he is roughly the same age as Zuko, which explains the apparent missing generation in this relation.

Although Aang seems at first to be Zuko's pseudo-grandfather as a result of this, there is, strictly speaking, no blood relation between the two boys, as Aang is genetically unrelated to Roku. But Uncle Iroh posits that this unique lineage marks Zuko with powerful significance, suggesting that being descended from someone carrying the Avatar spirit links a person to it in some way. The revelation is perhaps less Earth shattering for Aang when he eventually learns about it, but it impacts him nonetheless, deepening his inextricable link with the prince and further grounding him in the present day. More broadly, given the acrimonious end of Roku and Sozin's friendship and the disastrous effects thereof, the bond forged between Aang and Zuko gains new significance as a mending of past mistakes.

In the tradition of fantasy, but unusually for a children's show, the world of Avatar: The Last Airbender has a long, intricate backstory. Linking both Aang and Zuko to Avatar Roku ties the two main characters not only to each other but to that venerated past, even more so than the considerable amount that they already were. And while its sequel series, The Legend of Korra, would consciously move away from this focus, the reverence with which The Last Airbender treats history enriches its already deep lore which is so widely beloved.

Next: How Legend of Korra's General Iroh Is Related To Zuko's Uncle