Kimetsu no Yaiba/Demon Slayer Isekai. Female lead.

Rengoku, Please Don't Die.

"I am the flame hashira, Rengoku Kyojuro." Rengoku moved himself protectively in front of Haku and Tanjiro. "And I'm Akaza." The pink haired demon replied. The young Kamado had turned himself over and was glaring at the upper moon. Haku placed a hand on Tanjiro's back. "I know you wanna help, but you have to stay down or you'll die." Rengoku agreed with a quick nod then returned his attention to Akaza. "Why would you attack an injured person before me?" "I didn't want him to get in the way of our little chit chat." He then looked at Haku. Thw white haired woman was interesting. She did not appear to be a demon slayer, yet she was able to block his attack. She also smelled quite nice. he knew Douma would eat her in a heart beat.  "And I do hope the woman stays out of it from now on." Haku scoffed and stood up. One thing she knew for sure was he wouldn't harm her. It was against his morals to kill or eat women. "I have a name and it's Namikaze Haku!" Akaza was slightly shocked at the way she spoke to him. Like she had no clue he was an upper rank demon who could kill even this hashira. Rengoku took the moment to rush in and attack. Akaza easily parried his sword, then it was nothing but flames and blue as their battle began in earnest. Inosuke returned from helping the man that stabbed Tanjiro and looked at the battle frozen in shock. "There's...no opening to join Bug Eyes." Haku was frozen as well. The intensity of such a battle wasn't even comparable to reading it in a manga. A cold sweat covered her body. This wasn't a story anymore. It was real, and if she didn't think of something Rengoku was going to die! The battle had moved into the nearby forest and flames and clouds of dust emanating from the brush confirmed it. 'Rengoku please be alright.' she clasped her hands together in a praying motion.

Then Rengoku came flying from the forest slamming with a terrifying thud into the side of the derailed Mugen train. Akaza emerged not long after. "Come on Kyojuro! Become a demon before it's too late!" He was almost pleading with the hashira to accept his offer, but Haku knew he would not. Not ever. She had no time to think about the repercussions of altering the future. Why else would she be here anyway? She rushed forward, adrenaline pumping, into the fray. "Haku-san! Don't! " she heard Rengoku call out in a harsh tone that almost made her obey, but she kept moving until she was between him and Akaza. The upper moon simply narrowed his eyes, unsure of what to expect. He hadn't expected her first interference either. Haku breathed in deeply. "Do you remember the name Koyuki!" "What are you-" Akaza started to say before his eyes widened. That name. It meant....something to him. He was frozen, his mind and body fighting itself as the name rolled off his tongue in a hushed tone to himself. Rengoku had made his way beside Haku. He looked at her with narrowed eyes. What did she know? He would be taking her to the Master if they survived this ordeal. Without further hesitation, he took advantage of the demon's disoriented state. "Flame breathing, fifth form, Flame Tiger!" Enveloped in flames shaped like a tiger, Rengoku assaulted the demon before him. Both arms vanished then the blade met Akaza's neck, cutting through muscle and bone until ...Akaza snapped out of his confusion and jumped back, barely managing to keep his head on. He seethed with rage, veins popping out of his head. He was healing much too slowly. Was it the shock of nearly being beheaded? His eyes rested on the small woman a little ways in front of him, the woman who had shaken him to his core with one word, one name. He had to admit she was quite beautiful. Her red eyes seemed to hold a bit of sympathy for him and maybe something else. Then he took note of the sun rising up from behind the train cars. He turned to flee into the refuge of the forest. Haku felt relief as he fled. Tanjiro was hot on his heels and threw his katana into the forest where she knew it had hit its target. She felt tears prick her eyes catching bits of Tanjiro's speech which she knew irked Akaza to no end.

Haku turned to the flame hashira, happy to see he was alive with only minor injuries. "Thank you Kyojuro-san. That was an upper moon. I can still feel my body shaking from that attack I blocked, and he even pulled back a lot before he hit." Rengoku's usual smile turned more serious. "Yes, but my question is how you know all that. And whatever you said to him to get that kind of reaction." Haku's smile also faded and she looked down at the ground. "It's.... Complicated, and .... Farfetched to say the least." Rengoku placed his hand on her shoulder. "I see.... You did well and I thank you for the help. Also I apologize in advance for my rude actions." "What do you-" her question was cut short as a rough hand struck the back of her neck and darkness overtook her.

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Akaza approached a well kept mansion in the city. He was here to report to Lord Muzan the aftermath of the Mugen train incident along with the status of his search for the blue spider lily. Both failures. He pondered turning tail for a moment knowing he may face death in response, but he could never escape since Muzan Kibutsuji's blood ran through his veins. He jumped up to the second floor and entered the precipice of the room, remaining mostly on the balcony. Before him stood his master in the form of a child. "Akaza" As soon as his master spoke his blood ran cold. "What of your search?" Akaza gulped before responding. "There is no sign of the blue spider lily or any information of its existence." He sensed his master's displeasure, but continued "and I was unable to kill the flame hashira." Pain. Instantly his whole body was consumed with immense pain. "Not only have you failed in your search, you failed to kill a hashira and was even wounded by a mere child. AKAZA." He could feel his organs rupture within him. "AKAZA '' His blood felt like knives in his veins, slitting him continuously from the inside. "AKAZA. What pathetic form of an excuse do you have?" Akaza spat up blood, his body was shaking and he didn't dare move from his prostrated like position. "There was this woman who called herself Namikaze Haku." Muzan narrowed his eyes "oh." And with that, the order to bring Haku to Muzan was made clear to all the upper moons.