In a show full of nuanced and well-developed characters, Zuko and Azula are among Avatar: The Last Airbender's most compelling foils and the perfect nemeses. The two children of Fire Lord Ozai are both introduced as antagonistic figures who complicate things for Aang and his comrades during their travels across the Four Nations. Yet, as the story unfolds, it becomes increasingly clear that the royal siblings couldn't be more different.
Relentlessly searching to find and capture the Avatar, Zuko serves as the series' first real villain. However, it is later revealed that the Fire Prince was tragically banished from his home by his abusive father, who scarred his son for his perceived "weakness." Seeing the damage done by the Fire Lord's reign firsthand, Zuko eventually finds the strength and courage to change. Where Zuko slowly grows into a more becoming and honorable leader, Azula makes no such transformation, stubbornly supporting her father's violent tyranny to the very end of the show.
Making her first appearance in the closing moments of Avatar: The Last Airbender season 1, the Fire Nation Princess is enlisted to track down Zuko and their Uncle Iroh, so that they can be punished for their treasonous behavior. Unlike Zuko, who becomes more confident and strong with each human connection he makes, Azula grows increasingly erratic, even turning on her closest allies Mai and Ty Lee after they begin questioning the morality of what they are doing. It is her monstrous lack of empathy and narcissism that leads to her inevitable unraveling, seen so starkly at end of the series. Ultimately, it is Azula's isolation and inability to trust others that is her downfall.
Zuko, however, proves to be an incredibly empathetic and sensitive person throughout his arc. His banishment occurs after he expresses his horror that his father was willing to ruthlessly sacrifice patriotic Fire Nation soldiers to attain a military advantage. The prince routinely displayed his softer side throughout the show's run, defending a boy and his family from a cruel Earth Kingdom mercenary and forming a romantic relationship with Mai. Most notably, Zuko's relationship with Iroh is one of the show's most touching and a serves as the catalyst for much of his introspection and redemption. Then there is, of course, his willingness to become Aang's firebending master and joining Team Avatar, a true testament to Zuko's willingness to put aside his titles and familial obligations to do the right thing.
Whereas Zuko is able to see shades of gray, Azula sees the world divided strictly between black and white, a result of her having been so surrounded by and accepting of the Fire Nation's propaganda. Azula borrows from the template of her father, believing that fear and force are inextricable from being the perfect Fire Nation heir. Applauded by her father for her prodigious firebending gifts while her brother was routinely ridiculed for being less skilled, Avatar: The Last Airbender shows that Azula was conditioned early on to revel in the pain of others as it helped secure her elevated status. While Azula remains poisoned by her toxic upbringing, Zuko's determination to break free of it is what truly sets him apart.
In many ways, Zuko and Azula serve as parallels for Iroh and Ozai, respectively, who had their own conflict a generation earlier. Years before the events of Avatar: The Last Airbender, Iroh, a powerful general and heir to the throne, lost his son, an event that led to him leading a more compassionate and spiritual life. Zuko's own journey toward enlightenment similarly began with loss, after having his mother and home taken away from him in a short span of time. Like Ozai, Azula passes her brother in the line of succession and seems to have no qualms with the violence that comes with the Fire Nation's imperialism. When Ozai is eventually defeated by Aang, Zuko, like his wise Uncle Iroh, knows that there is another way, making it his mission to rebuild the world and break the Fire Nation's vicious cycle.