Kimetsu no Yaiba/Demon Slayer Isekai. Female lead.

Become a Demon Slayer; The Ubuyashiki Manor

Haku was floating in a dream. Before her was a sun and moon, forever opposites separated by the fine lines of dawn and dusk and in a never ending cycle. Twin figures stood on their respective sides moving in ways she instantly recognised. Sun breathing and moon breathing. She was mesmerized taking in both forms, ingraining how they were performed in her brain. Then the man on the moon side vanished while the one on the sun moved closer to her. Before her stood Yoriichi Tsugikuni, the first breathing style user and creator of sun breathing. Then the other must have been Michikatsu, his twin brother, who was now the demon Upper Moon One Kokushibo. She felt sadness at the thought. Jealousy, hate, resentment, all driving the birth of Kokushibo, but what she just saw was the truth that everything was a never ending cycle connected to one another and only existing because of the other. Yin and Yang. Onmyo. " Yes. You seem to understand it. As I told my brother, those who would surpass us uttered their first cries when I died. And you are here for a reason. Change the lives of the children who suffer because I failed to kill Kibutsuji Muzan. The balance you seek will be your own breathing style. My spirit will be with you until the end." Haku was unable to reply as the world once again became dark.

She could hear voices behind her and smell wisteria in the air. Her eyes fluttered open. "Ah she's awake." She heard a female voice. Turning her head she found it belonged to the insect hashira Shinobu Kocho. " She's so cute!" The next to speak was Mitsuri Konroji, hashira of love. In fact, all of the hashira were before her. She looked around, taking in her surroundings. Demon slayer headquarters, the Ubuyashiki mansion. Her eyes first locked onto Uzui Tengen. He was just as handsome and well flashy as depicted. "Oh, I think she likes me! Showing up outta nowhere and facing an Upper Moon head on, very flashy!" She quickly turned her eyes to the other hashira, not even acknowledging Uzui's comment, to which he just chuckled. Every one of them was as amazing as when they first appeared. Sanemi moved towards her like he was going to kill her and Rengoku intercepted him. "What I wanna know is why she knows about demons and in detail Is she some spy for Kibutsuji Muzan. If she doesn't talk, I'll make her!" Rengoku agreed. "I do as well having seen it for myself, but we should wait for the Master." Stone hashira Gyomei Himejima cried and prayed in his usual way "Child, what is your name?" Haku turned to the hashira and sat on her knees. "Namikaze Haku. Please just call me Haku. I'm not used to being called my surname in America." Uzui piped in after hearing America, "Ohhh! So we have a flashy foreigner!" Obanai the snake hashira waved his hand "Doesn't that make it more suspicious." There was a silence among the gathering until twin voices announced the arrival of the Master Ubuyashiki Kagaya. He looked even sicker than you could have though, but the air of grace around him was ethereal. All of the hashira instantly dropped to one knee in reverence, and Haku couldn't help but turn and bow. "Lift your heads my children, young visitor. I am the leader of the demon slayers corps and master of this house Ubuyashiki Kagaya. You may call me Kagaya" Haku lifted her head while remaining seated. "And I'm Namikaze Haku, please call me Haku, Kagaya-san." her body felt light and warm. Tanjiro wasn't kidding when he said his voice was soothing. She was able to relax. "Now then Haku-san. I would appreciate you telling us how you became so knowledgeable about demons, particularly Muzan Kibutsuji and his Upper Moons." Haku inhaled deeply. Surely he could believe her, or at least put enough trust into her to accept it. "Kagaya-san, I will tell you everything, but I would prefer to speak to you privately. The information I have is delicate and I can't say it's a good idea to give it out without some thought." All of the hashira seemed to mumble slight disapproval, particularly Sanemi. Kagaya closed his eyes for a moment. "I accept. I would like to ask you a favor as well. Would you join the Demon Slayer Corps? I will not have you participate in the final selection. I would like to assign a hashira to train you. I will also require you to have a hashira accompany you at all times." Haku's eyes widened slightly. Become a demon Slayer? In a way it was like a dream. She also worried as she now saw this as a new reality, and she wanted to change things if she could. She remembered her dream in which she spoke to Yoriichi. Her own breathing style and helping change the outcome was why she was here. "I'll do it! I'll become a demon slayer and I'll tell you everything I know." Kagaya smiled, pleased to hear her conviction. "Now then, would any of my hashira like to train Haku-san?" Rengoku was the first to step forth. "I would be glad to train Haku-san!" Mitsuri raised her hand "Oh, Me! I wanna train Haku-san!" Gyomei spoke next. " I will train the child well." Uzui chuckled "She's pretty flashy and cute! I wouldn't mind at all!" Sanemi scoffed, "Let me train her." The other hashira remained quiet. Kagaya motioned to the ones who had spoken up. "Now Haku-san you can choose one of my children who was willing to train you." Haku looked at 5 hashira who were willing to train her. Sanemi was out. Cute little angry pomeranian that he was, he'd likely kill me for what I know. Uzui would be ok, and maybe some ninjutsu from his kunoichi wives may be helpful, but Sound breathing just didn't seem to align with her and what she would create. No, the best option would be Rengoku. Flame breathing was a solid follower of sun breathing. It would be perfect as guidance to the balance she would need. "Kagaya-san. I'd like to train with Rengoku Kyojuro. I'm already acquainted." Kagaya nodded. "Good then. For now I will have you go with Rengoku. I will send a message with your assigned kasagai crow with more on your sword and uniform as well as when we can discuss matters of what you know. Farewell for now Namikaze Haku." Haku bowed one last time and stood up. Rengoku walked up beside her. "Well then. I will train you well, Haku-san. Come, my father's estate is close by. We can stay there." Haku agreed. "Thanks Kyojuro-san"

After a while they arrived at the Rengoku estate and Haku was ushered inside. Young Rengoku Senjuro came forward and bowed. "Hello, I'm Senjuro. Thank you for helping my big brother." Haku waved her hands. "Oh, no I didn't really do anything. I'm just glad everyone was safe." He was adorable. A mini Kyojuro. Haku wanted to leap out and hug him, but knew it would be strange for the boy. Rengoku showed Haku to her room. A simple futon and a desk. She gratefully went in and laid onto the futon. She was exhausted. This whole getting isekaied thing was a lot. She closed her eyes and within a moment was drifting off to sleep.