In Avatar: The Last Airbender, Fire Nation sibling royalty Zuko and Azula have been at each other's throats since they could talk. Their strong but toxic bond is formed from a lifetime of being isolated together from the outside world. But what if their connection is a lot deeper than originally thought? Reddit user u/EdBoi007 raises this taboo question in a "strong theories" thread in the r/TheLastAirbender subreddit.
Azula has seemingly hated Zuko since she was a child. Constantly picking on her older but weaker brother, he is the object of her torment and sadistic pleasure. Although it's an outdated and frankly problematic concept, it is said that kids tease each other to hide their more genuine feelings. Could this possibly have been the case with Azula and Zuko?
In the end, Zuko was the only person Azula truly had in Avatar: The Last Airbender. Even though he favored Azula, Fire Lord Ozai was more concerned with being the most powerful person in the world than being a real father to her and Zuko. Their mother Ursa was terrified of Azula's power and sociopathic tendencies, so she always took Zuko's side. This unfortunately made Azula think her mother hated her, and they were emotionally distant up until her sudden disappearance. One would assume that since Zuko's father hated him and Azula had no relationship with her mother, the siblings would turn to each other to get what they lacked from said parents -- which already sounds pretty Freudian in itself.
Azula's merciless teasing would go as far as roping in their Uncle Iroh to get a reaction from Prince Zuko. Even Iroh, who was objectively stronger and much more of a threat than Zuko, meant nothing to Azula. It was always her brother she had her attention on, whether negatively or not. This may be the only way she knows how to connect with Zuko, since she lacked proper socializing and nurturing from her parents growing up. A romance between the two is not too far-fetched of a claim either, especially considering the recent speculation that the Fire Nation royal family came from a long line of inbreeding. Perhaps all Azula wants to do is carry on the family tradition.
Another point u/EdBoi007 makes is that Avatar: The Last Airbender is based on Japanese anime, and the trope of the younger sister being infatuated with her older brother isn't exactly a new concept. In fact, laws against incest are nowhere to be found in Japan and, as a result, it is more accepted than in Western nations. Since the world of Avatar is based on historic Eastern countries, this indeed could be proven true. Especially considering Azula and Zuko are royalty, oftentimes intermarriage was common in order to "preserve the dynasty".
In Book 3 Episode 1 of The Last Airbender, Zuko barges into his sister's room and demands to know why she gave him credit for killing the Avatar. She explains that she did so as a "generous gesture" to repay him for his help. After a bit of prodding, she circles around him like a predator cornering its prey and reveals her ulterior motive -- if their father discovers the Avatar is alive, all of his glory would turn into great shame. She then crawls back to bed and bids a goodnight to her Zuzu.
Many Avatar fans compare this infamous bed scene to Cruel Intentions, a notorious film about two step-siblings who are too comfortable and wager the reward of sleeping with each other for a bet. This scene in particular seems to be the starting point of all speculation of what fans refer to as "Zucest".
Azula gets the most enjoyment out of toying with Zuko and has no regard for whoever else is affected by her doing so. Despite her constant torment, she does show him some kindness after she, Mai and Ty Lee took down Team Avatar with the Dai Lee. She offers him a way back into the Fire Nation by giving him the credit for defeating the Avatar, a kindness she shows no one else in the series. Zuko accepts this offer and returns to the Fire Nation.
Still, as mentioned above, Azula reveals her ulterior motive -- and that she was still manipulating her beloved brother. Azula's life seemingly revolved around him, so it would be no surprise if the speculation is true. Nonetheless, it would be hard to confirm any of this; it is a children's show, after all. Each one of these scenarios could easily be interpreted as a strong sibling rivalry instead. Yet, Grey DeLisle choosing to deliver Azula's lines with a certainly seductive flavor may make Avatar fans consider otherwise.