ATLA Soulmate Flower Scars AU where if one person gets injured their soulmate sprouts flowers on where they were injured but the catch is that they only sprout the flowers after they met their soulmate, even if its briefly.
So when the Gaang met Zuko for the first time at the South Pole Katara feels the briefest amount of pity for his soulmate due to the giant scar on Zuko's face, but that inevitably gets washed away by the events that follow.
Sometime later during his search for the Avatar Zuko suddenly sprouts flowers on his hands, and is quick to get rid of them, not allowing himself to think on who he has met so far and about his possibly injured soulmate for fear that it'll consume him.
Meanwhile after Aang burns Katara he is shocked to see that no flowers burst from his hands, but after the resulting argument from Sokka and seeing Katara heal herself Aang convinces himself that maybe due to Katara healing herself the flowers never had a chance to sprout.
They carry on like this, until the Catacombs where Azula shoots down Aang. After bringing Aang to safety and managing to save his life Katara notices that Toph has been suspiciously silent and very guarded about her back, and then after settling down at Chameleon Bay Katara saw Toph ripping flower petals form her back, understanding immediately.
Toph, sensing her, immediately asked what was happening to her. It then hits Katara that Toph's parents never told her about the flowers, which has Katara remembering her own mother and the stories of the flowers that bloomed on her told as little love confessions and a part of Katara burns a bit more, both in her grief about her mother and the fact that Toph's parents never bother to tell her about it.
Toph is understandably dumbstruck, almost to tbe point of denial but finds herself again, and decides to just keep this between herself and Katara.
Aang obviously remains oblivious to this fact after he wakes up, Toph refusing to say anything and Katara refusing to break her promise to Toph, and it remains thay way until Zuko accidentally burns the bottoms of Toph's feet, and Aang stumbles, discovering the rows of flowers had blossomed on his heels.
It creates an even more tense environment, with Aang confused, Toph stone-faced, Katara furious and Zuko even more ashamed.
They get past this, however, with Aang and Toph coming to a mutual understanding and Katara finally forgive Zuko. During their time on the Ember Islands, when its just Zuko and Katara they have an honest talk about soulmate flowers
Zuko reveals that traditionally Fire Lords never married their soulmates ever since Sozin's reign, thay hid Uncle Iroh was a key exception. Katara talks about her parents flowers, about how during hunts their mother would wait by the fireside waiting to see if anything ever sprouted, and if anything ever did she would tell heroic stories about how their father got those injuries when in reality the flowers were results of simple mistakes.
Katara then asked Zuko about his soulmate, to which he replies that he doesnt know who they are. That he only had flowers spout once, but never again. Katara then opened up that she never had flowers sprout, and that she's a little jealous because everyone else in their group knows that their soulmate exists except her. Zuko tries to reassure her, that maybe she simply hasn't met them yet. Then he cautioned Katara to be careful what she wishes for.
Katara didn't really take him seriously, trying to focus on the battle ahead and not on her feelings or the flowers. That is until the day of the final, during the Agni Kai and Katara felt flowers bloom at the center of her chest matching perfectly with the devastating wound Zuko took to protect her from Azula.
