(A/N: Ok, so, like I said, I'm going to try and provide some of the key details I remember about the theory here for you. These are just some of the things I remember the theory addressing. I know I'm not going to do the theory justice, or explain as well as the person who came up with it did, but I'll do my best. I'm sure I'm also forgetting several other details, but it's been almost a decade since I read the theory, so I'm actually surprised that I remember even this much.)
Why with Zuko? – It's been established in LoK that Aang's family wasn't the picture, perfect fairytale the world thought it was, and there were a lot of problems and dysfunctionality within it. His Avatar duties also came first, so later in life he ended up neglecting his family at times and had to leave them behind (also confirmed). With Katara being the Avatar's wife as well as the "princess" of the Water Tribe, she had to often stay behind to handle diplomacy in the Fire Nation and act in Aang's stead while he was off doing Avatar stuff. The theory examined the possibility of a full-on affair that happened between Katara and Zuko as a result of the two of them growing closer together with the more time they spent together. It's an established fact that Katara and Zuko had the strongest connection (even if not a romantic one) out of anyone else in the series. There were sparks between them throughout the series too, the writers themselves said this, and they even put in Zutara moments to keep shippers invested, not to mention there were other members of the creative team that shipped them together and put stuff in there, so there was actual stuff in the series, and these things weren't just seen through a shipper lens, which is why if an affair happened with anyone, it would be with Zuko.
Reason for the affair – The theory stated that one of the problems that may have taken place was Aang not being very experienced or knowledgeable in bed since he was raised by monks, and since he remained childish into adulthood (the part about his behavior was confirmed), so he was a lousy lover, so Katara was often left unsatisfied. And with Aang busy with his duties and Katara left behind, her connection to Zuko grew, as did those sparks. Plus, it has been confirmed that she has a thing for "bad boys". Whether the part of Aang being a bad lover is true or not, the theory states that the main reason Katara was went astray was due to the neglect she also faced from Aang being away so much, resulting in her feeling lonely and emotional. It also brought up other emotions and complications that would arise should Aang and Katara have trouble having children, by reason being for Aang being frozen for so long, which was confirmed to have affected his body. With the need to bring more Airbenders into the world, the theory points out how it's a little suspicious that they had 3 children so many years apart and not as many as possible closer together in age. The complications caused distress and emotional turmoil, especially in Katara if Aang was away a lot. And so something happened at least one time between her and Zuko, resulting in Bumi. The theory also provided alternate scenarios of it simply being a one-night stand out of feeling lonely on Zuko's part without Mai and lonely and neglected on Katara's, leading to a one-night stand. For the sake of this story, I used the scenario the theory provided of the feelings of neglect and loneliness going with the complications Aang and Katara might face from trying and failing to conceive, resulting in a drunken night where Zuko and Katara ended up falling into bed together after drinking too much. All the scenarios are realistic and happen in the real world, but for the sake of the characters, I thought this was the best and most realistic scenario, hence why I used that one for this story.
Before and after the affair – As I mentioned above, there's a lot of possibilities for how the affair went down. It could have been a brief fling or an on and off thing. It could have been a one-night stand stemming from a time of loneliness and weakness. It could have been after they got drunk one night and they barely remember what happened, which was the scenario I used for this story. There's lots of ways it could have gone down, but it did. Katara felt very guilty about it, and this affected her life and her choices afterwards. Note what became of her compared to the other members of the GAang. Everyone else got lots of praise over time and became known throughout the world for many deeds, but Katara not so much. In fact, she's the only member one of the team who doesn't have a statue of her. She's most known for being great healer, when she originally was opposed to the idea to women being stuck as healers, even fighting Pakku against it, only to end up in the very role she was opposed to, as well as being stuck raising the kids. Her guilt and mistake for the affair caused her to become less ambitious and less adventurous, and take such a backseat to things, and it's also why she never stepped in to address Aang's behavior to their children, because there's no way she wouldn't say or do something without there being a very good reason behind it. After seeing all the potential Katara had and what her character is like, why would she just become a baby making stay at home mom who doesn't get involved in things to the point where she doesn't get a statue and is really only known mostly for being a healer, and why would she never do anything to fix the problems happening with Aang and her children?
Keeping it a secret – The theory gave two different scenarios about keeping the affair a secret. One of Aang finding out, and one of him not. If Aang found out about the affair, they would have all decided it was a mistake and moved past it. And with Aang's philosophy of forgive and forget (include him telling Katara to forgive her mother's murderer and forget about it), they put the affair behind them (as evident by them still being together in the show). It would also have to remain a secret, including Katara's pregnancy by Zuko. The world was still having a shaky peace. To reveal that the Avatar's wife had an affair with the new Fire Lord, even if it was a one-time thing, would cause a massive scandal, and could even restart the war. For the sake of the world, they all kept quiet about it, and pretended that Bumi was Aang's son. The other scenario is that Aang never found out, which I felt was the more realistic version, and why I used it for this story. An alternative was that Aang suspected, but never said anything because he didn't want to know the truth, and chose to believe in the best of Katara and Zuko. The theory also included the two different possibilities of whether Katara ever told Zuko the truth about Bumi. This really could have gone either way, as whether Katara told Zuko or not, neither would have been able to reveal it to anyone else. For the sake of this story, I had Katara chose the safer scenario of not telling him so as to better keep it a secret.
Bumi's name – This I always found a little peculiar, even without the theory mentioning it. While King Bumi was a good friend of Aang, neither Katara nor Sokka really cared for him, especially since he had nearly killed them with his eccentric games and such. He made them uncomfortable, and at most they tolerated him and respected his position and power. So why would Katara let her son be named after such a person she didn't care for? It's because she felt guilty over the affair and let Aang choose the name without complaint since she felt she owed him that much. Yes, Aang may want to name one of his kids after his only still living old friend, but can you really see Katara wanting to name one of her kids after Bumi of all people? Maybe Gyatso, but Bumi?
Aang's neglect and Bumi's personality – The theory points out how it was established in LoK that Aang really wasn't the best father. He was very neglectful and distant with his children, while showing clear favoritism to Tenzin and giving him special attention because he was an Airbender. And when he wasn't with him, he was off being the Avatar or founding Republic City. In the scenario of Aang finding out about the affair, Aang may have unintentionally been extra neglectful of Bumi since he knew Bumi wasn't his son, or maybe he deliberately neglected him because it was painful for him knowing Bumi wasn't his, as Aang is known for running from his problems. As such, he may have also been neglectful of Kya too because that was simply the behavior he had adapted as a father. Tenzin didn't come along until a while later, so Aang was already being neglectful of both Bumi and Kya for years before Tenzin's birth, so the whole focusing his attention on Tenzin wasn't an excuse yet. This is where Bumi's personality comes from. Personality is not inherited or passed down from parent to child. Bumi's behavior is erratic, childish, and attention is very common behavior of children who are neglected by one or more parent, or they may imitate the parent they're trying to get the attention of. Since he was extra neglected by Aang, not knowing Aang wasn't his real father, he often sought his attention and tried to be like him, copying his behaviors and mannerism to an even greater level until that became his natural personality. Bumi was more neglected than Kya too, so Kya didn't experience as much neglect. Or in the scenario that Aang didn't find out about Bumi, then he was just always neglectful, even before Tenzin, because he simply didn't have time for his kids. Whatever the case, it's still true that Bumi faced the most neglect, even some from Katara. And for what reason would Katara have to neglect Bumi? If anything, she would likely pay him more attention, but she didn't, and focused more on Kya. Yes, them both being Waterbenders would help them bond, but she wouldn't neglect one child over another without a good reason, and him being a nonbender isn't one.
Bumi's appearance – Bumi has been called the black sheep of the family who is different from his siblings in many ways. The theory mentions that this is because he has a different father. Even his appearance is different from Kya and Tenzin. Kya looks Water Tribe, taking after Katara, Tenzin looks Air Nomad, taking after Aang, but Bumi looks very Fire Nation. He has a lot of Fire Nation features, with some Water Tribe mixed in. Seriously, look at him next to other people in the Water Tribe, the Air Nomads, and the Fire Nation. He looks very Fire Nation and has many of their features. He looks a lot more like Zuko than Aang too. Not just him, put him next to the likes of adult Zuko, Iroh, Zhao, Sozin, and other Fire Nation men and say he doesn't look like them, especially compared to Air Nomad men. If you didn't know who he was and had to place him in either Air, Fire, Water, or Earth, wouldn't you honestly be able to say that he doesn't look like he belonged to Fire? He got his a slight skin tone and eye and color from his mother, but otherwise he looks Fire Nation. Yes, in the family picture when he was a kid, he may have looked a little like a young Aang, but he also looks like a young Zuko. This is actually true of all young kids in ATLA and LoK, as their features don't become more defined or stand out as much until the characters become older. And older Bumi looks a lot like older Zuko, but nothing like older Aang. The person who came up with the theory mentions how they doubt this was intentional on the artists' part, but the same can be said for Suyin looking a little like Sokka and kicking off that theory.
Bumi being a nonbender – I guess this makes this theory two in one, but it points out the possibility of parents from opposing element nations may increase the likelihood of having a nonbender child; I mentioned this in the story. Bending can be inherited, the child taking on one of their parent's skills. But bender parents can have nonbender children. It's possible that if two parents are opposite benders that their opposing elements can, shall we say, cancel each other out, and increase the likelihood of having a nonbender child. Again, this is another theory entirely, but if there's any legitimacy to it, then it only supports this theory further. Bumi may become an Airbender later, but that was due to the Harmonic Convergence that made thousands of other people of different nations Airbenders. Kya gained her Waterbending from Katara, and Tenzin got his Airbending from Aang. But as we saw with Tenzin, bending abilities seem to be extra strong in his bloodline since all 4 of his children are Airbenders. This seems to suggest that as the Avatar, Aang's descendants are more likely to be benders, particularly Airbenders since that's his nationality. This is possibly true of all the descendants of Avatars that bending is strong in their family until the bloodline gets watered down enough, as even all of Roku's descendants are powerful benders. This only makes it stranger that Bumi is a nonbender if he were Aang's son. If there's any truth to this theory, it only supports the main theory.
Bumi's position – The theory mentions how Bumi was the commander of the Second Division of the United Forces, which is primarily Fire Nation. Realistically, it's almost strange that someone as wild, eccentric, and undisciplined as Bumi would become a commander. It wouldn't be surprising that since Bumi was actually Zuko's son that Zuko pulled a few strings to help get Bumi a high-ranking position if he knew or at least suspected. It would also allow him to be closer to his son since the United Forces was mostly Fire Nation and primarily in the Fire Nation.
(A/N: Well, these are all the points I remember right now, but I know there were more, and they were much better explained than what I did. And remember this is just a THEORY. Can you argue against the points of a theory, of course, because it's not an established fact, which is what makes it a theory. But it still doesn't change the possibilities of these indications being true. Maybe they are, maybe they aren't, but even if you can argue them away, they still back up the theory and give it legitimacy. Again, when I came across the theory, I basically laughed when I saw it was about, but couldn't believe how much sense it made when it was all laid out. And the more indications there are, the harder it is to dismiss it. Sure, you can probably call one or two things a coincidence, but eventually there are too many factors to dismiss, and you have to consider the possibility, because it COULD have happened, even if you have no concrete proof, because again, that's what makes it a theory. The theory of Sokka being Suyin's daughter is another fan-theory too, hence why I included it in the story since it went along with it. And there are crazier theories out there, like Tai Lee being an Air Nomad survivor, Yue sending a meteor to Sokka to make into his space sword, or Azula's insanity being due to inbreeding in the family. They're things that are possible because they could have happened. And the theory that inspired this story legitimacy to it, and evidence to back it up. Now, I'm not saying the theory is true, but it was eye-opening. So, if you haven't reviewed yet, now's your last chance. But remember, if you're just some angry shipper who's going to throw a tantrum by ranting and throwing insult, you'll just be wasting your time by reviewing, because your review will be deleted once I realize what it is.)