Zuko's transition from a main baddie to a crucial member of the Aang gang is a big part of what makes Avatar: The Last Airbender so special, but Zuko's relationship to Aang goes much deeper than becoming one of his best and closest friends. The fact of the matter is, Zuko is Aang's great grandson, a revelation Aang is not made aware of until Avatar: The Last Airbender: The Promise Part 3.
With its recent addition to the United States' streaming catalogue, fans both new and old are binging The Last Airbender on Netflix and looking for more -- and luckily a long slew of graphic novels pick the series up right where the show leaves off. Avatar the Last Airbender won The Peabody Award in 2008 specifically for its "multi-dimensional characters, unusually complicated personal relationships for a cartoon serial, and a healthy respect for the consequences of warfare," which are all qualities that the comics definitely keep true to. Kicking off the series of graphic novels is The Promise, a story in which Aang and Zuko are at odds yet again, this time with the morality of each heavily in question.
As the two friends are left conflicting with each other over the fate of the new world, Aang is left to contemplate what he is doing. Aang has made a promise to his friend Zuko, a promise that ultimately he will kill him if he believes Zuko to be turning evil again, and at this point neither is sure which of them is in the right. When he brings this challenge to his past self Roku, who comes forward in the form of a force ghost, Roku fills Aang in on a little secret about his pal Zuko. It is here that Aang learns of the fact that Zuko is actually Roku's great-grandson. Since the Avatar cycle is a long line of reincarnation, according to the religion of the Avatar-verse, this technically would make Zuko Aang's great-grandson too.
This revelation that Zuko is actually Roku's great-grandson ironically has the opposite effect on Aang than he had intended, but in the end, that is a very good thing. While this revelation is not entirely new to Zuko (revealed to him in the television series), the news is never actually revealed to Aang or anyone else in the gang until this point, leaving many fans to sometimes glance over this stunning revelation. Adding this moment to the comics provides emotional weight to Aang's story, and even added weight to the show, now serving as a mirror to the moment in which the revelation helped Zuko find his path.
The two are later able to discuss this bloodline, bonding them even closer together as not only friends, but pseudo family. This discussion however quickly shifts attention and importance to the next Aang gang adventure in the larger Last Airbender lore, a question that the series itself left fans to wonder: What exactly happened to Zuko's mother?