The moment Zuko saw his younger sister defeated, chained up, sobbing in agony and breathing fire at him whenever he tried to get too close, he swore to himself that he'd never be at war with her again. It didn't matter what she said, what she did, or how useless his efforts seemed, he'd remain by her side, helping her heal, taking care of her, hopelessly chasing the dream that maybe they could learn to get along again.
At first, he thought it was all going well. Azula still looked too unmotivated and sad for his heart to truly be at ease, but she had surprisingly not rejected his help at any point. She allowed him to care for her in whatever way he saw fit, not objecting when he sent her to a psychiatric hospital, nor when he brought her home after five months of intense treatment. He wasn't naive enough to believe that she had truly understood the error of her ways overnight, but she was still making an effort to at least not pose a threat to him or anyone else anymore, and he took that as an excellent sign. Maybe, someday, she'd redeem herself in the eyes of everyone else and be allowed to just put the past behind her, like Zuko himself had.
Then, out of nowhere, she was gone.
She had taken no clothes, no food, no weapons, no nothing. Just got out of bed in the middle of the night and quietly disappeared.
Zuko had people searching for her in every room and secret passage of the palace, then turning Caldera city upside down. Searching in every island, town, village, mountain, cave, beach, woods and fields. At every home, dock, market, temple, boat, and carriage.
Even the other nations were all aware of his desperate offer to make the person who found Azula, and brought her to him alive and well, obscenely rich for the rest of their life. He had even promised that, if the person was a criminal and a Fire Nation citizen, all of their wrong-doings would be forgiven immediately and removed from all records.
Still nothing.
Tons of impostors, lots of girls being mistaken for her, but not a clue of where on Earth his sister actually was.
Even when chasing the Avatar, he had not been that out of his mind due to the sheer frustration of trying to find someone that seemingly could not be found on this world. It took all of his strength to resist the illogical urge to have icebergs searched, like Azula could have somehow accidentally ended up trapped in one like Aang was.
At first, the whole world was panicking as much as he was. They all knew that someone as dangerous as Azula walking around freely was a bad idea, especially when she had not recovered enough to not potentially think of anyone and everyone as a potential enemy that was out to get her.
But after five long years, no one else seemed concerned, believing that she had simply decided to renounce her old life completely, including her desire to cause them any harm - or that she hadn't yet turned up simply because she had been dead this whole time.
Zuko refused to acknowledge that possibility. His mind could not accept that someone as clever, strong, and resilient as his sister could have simply let the merciless waves of the ocean carry her away to her death, or that she had starved or perished due to an illness while on the run, or that her deteriorating mental health made her an easy prey for some former enemy or even a random, lone bandit that didn't even know she was a princess.
But as angry as he was at that idea, it did not get to him half as much as the rumor that Azula had never ran away at all, but had simply been executed in the dark of the night, on Zuko's orders, so she'd not pose a threat to his reign, but without him having to forsake his image as merciful, kind ruler.
He had not listened to his uncle's advice that making it illegal for people to say that would only make them all think it was true. Sometimes he wishes he had. Other times he was too consumed by his obsessive search to care about it.
He had often heard that the loneliness that came with missing someone so desperately made the world suddenly feel impossibly empty, that it made one's home seem much larger and quieter.
Azula's absence couldn't be more different from it. It loomed large over him every second of the day, a screaming reminder of his failure, both as a ruler who has to protect himself and others from a potential threat, and as an older brother who had the responsibility to help his sister recover. It was unrelenting, all-consuming, suffocating. Like the walls were closing in on him, about to crumble bury him under the debris, forever trapped. Every face taunted him with how different it was from the one he wished to see. Everyone's voice suddenly sounded so wrong, simply for not being Azula's.
Whenever it became too much, he'd go to her chambers and lock himself inside for the night. Once any trace of Azula's perfume was gone from it, he'd pour a few drops of it on the sheets and pillows. Once he ran out of it, he bought more of it - but just knowing it wasn't the one she had used nearly everyday was enough for his mind to play tricks on him, convince him that it wasn't the same, that there was some noticeable difference that only he could pick up on. Still he'd put it on her bed, on her clothes, and sometimes on himself, as it was familiar enough despite part of him screaming that it wasn't the same.
Sometimes he'd cry like a lost, lonely little boy, holding onto one of her pillows, his very soul aching as he dreamed a time long gone, when Azula was the sweetest little girl in the whole world, always wanting to copy everything he did and following him around everywhere, insisting on sharing his bed.
Other times he'd stay up all night, plagued by the most despicable fantasies that he couldn't purge.
His first night back home, all those years ago, when Azula was taunting him, toying with him, dangerously close to him as she revealed she had set him up by telling their father he had been the one to kill the Avatar. Him staying in her room instead of leaving, teaching her a lesson for lying to him yet again. Seeing her smug expression turn to one of obscene pleasure as he had his way with her.
Her coming back without warning after all those years, crawling into his bed in the middle of the night, blushing as she offered herself up to him in exchange for his forgiveness, promising to never leave again, kissing him deeply as they made love.
Her being brought to him in chains, defeated, apologetic, fearful, being confined to his bedroom for the rest of her life, learning to love the chains she knew she'd be freed of, always eager to please him.
Zuko resisted the urge to indulge in these thoughts at first, chastising himself, fasting, praying, speaking to doctors, meditating, abstaining from sex, looking for lovers that were nothing like Azula, looking for lovers that were exactly like her, trying to think of any other woman, trying everything in the hopes of healing from whatever sickness had pushed him to these unnatural inclinations.
But once he succumbed to them for the first time, touching himself in the dark, under her covers, desperately wishing he could touch her instead, it was impossible to keep denying himself that twisted pleasure. The shame never disappeared completely, but it became easier to ignore.
One day he accepted defeat. He'd never see his sister again. She'd never atone for her past mistakes, they'd never reconciliate, he'd never have her for himself. It was a bitter moment, but one that allowed him to stop acting like a mad man for a few weeks.
Until the news reached him: a pale, skinny, dark haired girl who was no older than 20 was running around through a small town in the Earth Kingdom - and used blue flames to defend herself when guards tried to approach her.
Zuko packed his things and left immediately.
For nine seemingly endless days he and his men tried to find her, with no luck. Azula had seemingly vanished without a trace once more.
On the tenth day, before the sunrise, he saw her, moving through the forest quietly and with unbelievable speed.
But she wasn't fast enough to out-run him.
He chased her by the riverside, doing his best to avoid the small blasts of fire she'd send his way, heart beating so fast it seemed as if it'd leap out of his chest at any second. He didn't take his eyes off her for one second, didn't scream so his men could follow the sound of his voice in case he needed back up, didn't bother to take in his surroundings, focusing only on what was on his immediate path - knowing that if he lost her now, he would likely never find her again.
Azula reached a cave, and went into the darkness without a care, running through the tunnels without ever hitting a dead-end, without slipping on the irregular ground, without so much as using her firebending to be able to see what she was doing. This had been her hiding place for a long time, maybe for all those years they spent apart.
Just as she was about to leap out of an exit that would lead to a lake hidden away by huge trees, Zuko managed to grab a hold of her, keeping his sister pinned to the cave-wall, holding her so tightly that she cried out in pain.
"Let me go!" she screamed, trying her best to free herself. She had even tried to breathe fire, but the chase had left her out of breath "Zuko, let go!"
"Shut up!" he made her back hit the cave walls again, face mere inches away from hers as he tried to get her to stop struggling. Once she did, he locked eyes with her, seeing the confusion, fear, and madness burning in them.
"Please... don't hurt me..."
He embraced her like he had been dreaming about for so long, feeling her body tense up, like she had turned to stone.
"Where were you? Why did you leave?" he demands to know, but all she does is cry and repeat her plea to not be harmed "I won't hurt you"
"Liar!" she screams, trying to push him away, but he isn't going to put up with it.
He grabs her by the hair and presses a searing kiss to her lips, ignoring the pain and the taste of blood as Azula bites the lower one, his tongue quickly managing to invade her mouth as her hands are clenched into fists, repeatedly hitting his chest over and over and over again.
Until finally, she stops resisting once more, awkwardly kissing back without seeing to know what she's doing or how to do it, letting him press his body to hers as he holds her in his possessive embrace, until she moans against his mouth.
At first, when he pulled away, Azula had the reaction Zuko had expected. Looking at him in shock, blinking repeatedly as she tried to process what in the world had just happened. He assumed she would then hit and insult him, outraged at what he had done. Or maybe even run away in panic, confused and disgusted by his actions.
But the sweetest smile he had ever seen begins to form as she looks at him, all teary-eyed, hands shaking as she hesitantly caresses his face - then laces her arms around his neck, pulling closer, holding tightly as he kisses her lips, face and neck with a kind of passion and joy that can only be described as delirious. And Azula welcomes it, like she had been waiting for this her entire life, like she has ached for this forbidden moment all those years, like she had been the one desperately searching for him.
He wants to question her again on why she left at all, to immediately drag her to his ship, keep her chained to his bedroom until they're home and let her know she'll never be out of his sight again, that he won't let her escape a second time.
But can't bring himself to. Not right now.
Because all that matters is that she's here. That he can touch her, hold her in his arms and swear to never let go, that he can wipe those knowing smirks or fearful expressions off her face with one kiss after another, that she gasps and blushes adorably when he feels bold enough to sneak one hand down her shirt, touching every inch of her soft, warm skin.
He tears her shirt off, not caring that this will look incredibly suspicious to everyone once they're leaving, marking her pale flesh with hickeys and light burns as Azula laughs and throws her head back, her clever hands already freeing his cock from pants, stroking him.
He kisses her deeply once more, then undresses her fully, pulling her to his lap as she makes him remove his shirt.
Once he's inside her he cannot bring himself to think if this is normal or not. If he's taking advantage of her mental instability. If her cries are of pain or pleasure or both. If she loves him as much as he loves her. If she'll regret this. If he'll regret this. If she'd want their first time to happen in a different way, at a different time and place. If word of it will get out to people none of his relatives, friends, subjects and allies will ever be able to see him the same way again.
He doesn't care. He wouldn't care if he is dishonoring them both, if he'll lose his crown over this. He'd even let Ozai burn his entire face if it meant he could still have this moment.
If it meant feeling Azula grinding on him, bouncing on his hard member, running her fingers through his hair as he kisses and bites her breasts. If it meant enjoying the lovely sight that is her trembling, holding onto him for dear life as he makes her climax, then laying down on the ground so he can claim her again, pulling him closer to herself and wrapping her legs around him, wanting to keep him above her.
Because he knows she'll never want to leave now that he has marked her as his.