Kimetsu no Yaiba/Demon Slayer Isekai. Female lead.

Tanjiro, keep a secret please.

Haku approached Sanemi who was waiting just outside the estate. She waved at him' "Hey! Finished my meeting. Ready to go?" Sanemi looked over at her in his usual irritated way and simply stood and started walking. 'Alrighty then.' Haku sighed inwardly and followed. After a while Sanemi stopped looking up at the sky. Night was falling, and they were long outside the safety of the wisteria growing at the Ubuyashiki mansion. He grumbled to himself and crouched down. "Get on. The last thing I want is to have your weak ass to deal with and a bunch of demons." Haku blinked, dumbfounded. He was going to carry her piggy back? She was not able to run as fast as the demon slayers yet, so it made some sense. With slight embarrassment she climbed on his back. The wind Hashira blushed as her chest made contact with his back. "All aboard the Sanemi express." Haku called out trying to break the tension. It worked, but earned her a tongue lashing from the hashira. "I'll drop you on your ass woman!" He had already taken off so Haku hoped he would not actually drop her. He was quite fast, and the wind felt good on her face. She let out a small giggle of amusement. Sanemi glanced back at her and found her smiling face quite pleasing. Wait. What was he thinking! He could not possibly be smitten with this anomaly of a girl. He quickened his pace, wanting to focus on travel and not the white haired beauty on his back.

The moon was high in the sky when they arrived at the estate. Haku's excited giggles had stopped and her head was nestled in the crook of Sanemi's neck and she was fast asleep. Rengoku was waiting at the manor entrance. "Ah, a successful meeting I see. I can take her inside." He pointed to the sleeping girl. Sanemi adjusted her. "I got it." He said nonchalant, and headed into the estate. Once inside he was unsure where to go. Rengoku reached and gathered Haku in his arms. "I'll take her to her room. Sanjuro will take you to your room. We have visitors in the room next to Haku-san." Sanemi eyed Haku while Rengoku spoke. "Visitors?" He questioned. "Yes, young Kamado, the boar boy, and the yellow boy are here." Rengoku knew Sanemi was not on good terms with Tanjiro. Sanemi scoffed and then followed the patiently waiting Senjuro to his room. Rengoku smiled down at Haku, smiling softly as he carried her to her room. He lay her in the futon and tucked her in. "Good night." He whispered softly before exiting the room.

When Haku woke the next morning she found the Rengoku household bustling. She walked into the dining area to see Tanjiro, Inosuke, and Zenitsu lively eating breakfast. Rengoku motioned to her and patted the seat between him and Tanjiro. "Come sit here Haku-san. As you can see we have visitors. They will be training here for a little while as part of rehabilitation. The butterfly mansion is quite busy these days." He explained this after Haku had sat down. Haku looked at the trio and smiled. "That's great! It'll be amazing to have some training companions!" Tanjiro agreed. "Yes. It is nice to see you again, Haku-san." Haku smiled. "Just Haku is fine Tanjiro. And likewise." She replied excitedly. Zenitsu stopped arguing with Inosuke and joined "Just call me Zenitsu, Haku!" He fawned in the usual way he did with women. Haku reached across the table and patted him on the head. He was ecstatic. "Marry me!" he exclaimed. Tanjiro gave him a mildly disgusted look, but overall everyone ignored the outburst and enjoyed breakfast. Inosuke reached over to snatch a rice ball off Haku's plate. "I'm taking this! Fight me if you want it back!" Tanjiro immediately scolded him. "Inosuke! That's not yours and that's rude!" Haku waved her hand. "Go ahead Inosuke. I'm done anyway." The bear headed boy felt bubbles in his head at her sweet smile and dug into the rice ball. Haku looked around. "Where's Sanemi-san?" She noticed the wind Hashira was absent. "He left first thing this morning. Missing out on this wonderful, lively breakfast though." Rengoku answered her. That was disappointing. She wanted to see him off.

After breakfast I would be training. Haku was heading out into the yard to join the boys only to be stopped by Rengoku. "Haku, I have a request. Please call me Kyojuro. No need for honorifics, just as you do with the young slayer trio." Haku noted he had dropped the honorifics for her as well. She smiled. "Absolutely, Kyojuro!" She headed out the door leaving Rengoku standing with a slight blush. After a moment he joined everyone and began instructing everyone. Haku was beginning to construct her breathing technique. She seemed to naturally fall into using total concentration constant. She attributed this to Yoriichi's influence from beyond. Having seen Sun, Moon, and Flame breathing up close, she was able to begin constructing her own style. She wanted to create a style of harmony between Sun and Moon. Tanjiro walked over to her. "Haku, that's not any breathing form I've seen. It seems to have elements of my Hinokami Kagura." Haku stopped swinging her sword. "Well, I just took the elements of a lot of styles to blend in harmony, so it's not surprising." She could not reveal her knowledge of sun breathing to Tanjiro, who was nodding at her explanation. "When I got here I had a fight with Reangoku-san's father because he said I was a Sun breather. I just wondered if you knew more since I could see Hinokami Kagura in your moves." Haku shrugged. "The Hinokami Kagura you use has served you well, so use it." she wanted to deflect the conversation. Tanjiro let it go and the two decided to train together.

Haku followed Tanjiro into the room he was sharing with Inosuke and Zenitsu. Tanjiro had said he wanted to show her something, and she hoped it was what, or who, she thought it would be. As soon as she entered the room Zenitsu was around her waist. "Haku. How are you tonight!" he cooed. Haku patted his head while Tanjiro peeled him off her. "Fine. Excited to see what Tanjiro is talking about." The yellow haired boy was flung across the room by Inosuke. "Quit hanging all over her. She can't walk like that!" He scolded Zenitsu. Haku reached and patted Inosuke's head as well. "Thanks Inosuke. He's not hurting anything tough, so I'm fine." Inoskue was instantly calm and bubbly again. Tanjiro cleared his throat. "Anyway Haku, I have someone I want you to meet." He was bent over a small wooden box. Slowly He opened the door. "This is my little sister Nezuko. She may be a demo, but I swear she has never hurt anyone!" Nezuko emerged sleepily from the box and crawled towards Haku in her tiny form. Haku smiled, sparkles all around her. Nezuko was adorable! The younger Kamado sibling sniffed the air and climbed into Haku's lap. Haku patted her head. "I believe you Tanjiro, so don't worry." Tanjiro was surprised at Nezuko. "Wow, she really seems to like you. Maybe it's your smell." Haku cocked her head to the side. "My smell?" She knew Tanjiro had a very acute sense of smell. Tanjiro waved his hands. "You don't smell bad or anything. Actually you smell really sweet, like flowers." Haku sat thinking for a moment. Would this be related to her blood? It smelt and tasted sweet. She glanced around the room, and Insouke and Zenitsu had fallen asleep. She was petting Nezuko as she rolled around on her lap. It had crossed her mind to ask this of Tanjiro, but she was unsure. She bit her lip and made her decision. "Tanjiro. I really feel I can trust you with this, so I am going to ask, but I need you to promise to keep it secret for now and also not ask why I know what I know. I will tell you Kaguya-san knows why and has agreed I should limit what I reveal for now." Tanjiro sat up straight, caught off guard by the seriousness of her tone. He nodded. "I can smell you're telling the truth and that I can trust you. Besides, Nezuko likes you." Haku giggled a little. "Alright. I need you to take some of my blood and send it to Lady Tamayo. I will include a note for her eyes only explaining why." Tanjiro's eyes widened a little and he went to ask a question and stopped himself. He nodded and dug around for a needle and syringe, supplies Tamayo had given him. He inched closer to Haku who extended her arm for him. A quick prick and Tanjiro had a little vial of her blood. "It smells sweet." Tanjiro commented and Nezuko seemed mesmerized by the scent. Both Tanjiro and Haku took note of the almost drunken state she was in. Haku slid Nezuko from her lap and scribbled a quick not for Tamayo, including the observation she made with Nezuko. A small meow was heard and a cat appeared. Tanjiro placed the vial and note in a pouch on its back. "Thanks Tanjiro." Tanjiro smiled. "No problem."

Chachamaru appeared before Tamayo. "Oh, what do we have here? From Tanjiro?" She opened the pouch and took a vial of blue colored substance and a note. She opened the note and it read.

' Dear Lady Tamayo,

My name is Namikaze Haku, and I cannot divulge how I know of you, but do know Kagaya-san is aware of my knowledge. Tanjiro was kind enough to not ask questions and allow me to send this vial of my blood to you. I am aware it seems strange, but my blood is blue. It also has a sweet smell and taste. When getting my blood, Nezuko seemed to be entranced and almost made drunk by the blood like many demons experience with a Marechi. I am not sure what this means for demons or humans, but I feel this could be important in stopping Kibutsuji Muzan. I am training to be a demon slayer and change the fate of humans and demons. Please study this and see if there is any reason for it being blue.

Thank you,

Namikaze Haku.'

Tamayo's expressions changed from shocked to quizzical as she read the letter. "Yushiro" she called her assistant. She would see what secrets this blood held.