Kimetsu no Yaiba/Demon Slayer Isekai. Female lead.

The Swordsmith village

Haku woke to find Muichiro and Tanjiro talking. She yawned and slid the little demon Nezuko that was sleeping on her arm off. She was sitting up groggily when she saw it. Hantengu, creeping into the room. She grabbed her sword but Muichiro was faster. "No! Don't!" She called but it was too late. Muichiro used his mist breathing to behead Hantengu, only creating a young and strong variation from the head and one from the body. Anger and pleasure were tattooed on the tongues. The one for pleasure, Karaku waved his fan at them, sending Muichiro flying. Haku jumped and grabbed Nezuko and Tanjiro away from the blast. Anger, Sekido, looked at Haku. "Lord Muzan is most displeased. I will take you back with me after we are done here." So Muzan had not found a way to completely avoid the sun yet. Her blood was still needed. They could still win, and they already had a cure to use against him and the others. Haku looked at Tanjiro and then the normal sized Nezuko. "Listen, we have to get to the main body. Cutting off the heads here will just spawn more of them. The one with 'fear' on the tongue is the real one." Sekido snarled. "You really are a nuisance." He lunged at Haku who blocked him with her sword and bounced back just as lightning erupted from the staff. She felt her body buzz with energy. If that hit directly, it would be bad. She knew she needed to get part of one of the demons to act as a barrier. A shot fired and the demon;s head was blown from its body. Crap. Genya had appeared and beheaded the demon who split off into another. "Genya!" the boy jumped at her scolding tone. "Genya, listen, beheading them won't work. They just split off into more demons. We have to get the main body. Tanjiro can sniff it out I'm sure." Tanjiro was already on it, sniffing the air and pointing in the direction of the main body. The demon with the words for sorrow on his tongue, Aizetsu, plunged his trident into Genya. Tanjiro screamed as Genya was thrown onto the ground. Aizetsu tried to remove his weapon but could not. He struggled with Genya. Nezuko had taken the fan from Karaku and sent him flying. When she turned to use it on Sekaido, he plunged his staff into her and let electricity burn through her. Tanjiro ran to her aid. Haku took the chance to cut off his arm. She threw the staff away and threw the arm at Genya. "Genya! Use that!" Genya seemed surprised but wrestled himself over and took a bite from the arm. The tips of his hair turned yellow as he demonized. He was then able to attack Aizetsu. Haku turned to Tanjiro. "Go after the main body now! Make sure the tongue says 'fear' on it. One that looks just like it will have 'resentment' and just create another if you attack it. You're gonna need Hinokami Kagura to behead it." Haku was slammed to the ground by Karaku as he returned. She writhed underneath him as he laughed. "Come on little girl, can't do any better than that?" Haku took that as a challenge and wrapped around the demon. "Inyo breathing, with form, Tori to Uke!" She threw the demon down, snapping his neck and breaking his arms. She jumped up and immediately slashed off his legs and one arm. She took hold of the one arm and ran over to Nezuko who was struggling with Sekaido and used the hand to grab the staff. "Nezuko, if you use this he can't shock you." she let Nezuko take the arm from her and she got in a quick slice to the demon's achilles so he fell to the ground. Nezuko smashed his head, but made sure not to behead him. Haku's muscles burned from using her breathing style, but she kept moving. Gebnya was holding his own against Aizetsu, so Haku focused on Karaku, which had now healed. All of the demons suddenly started to decay. Tanjiro must have been successful. Haku turned and left the others and headed to where Muichiro would be.

When she arrived, there was a thick mist over the area. Gyokko's scream told her the battle was over. She rushed forward to cat Muichiro as he fell. Muichiro looked up at her. "An angel? No, I'm not dead. Haku?" Haku blushed. "Yeah it's me. We managed to take care of our demons." She helped Muichiro up with the assistance of Kotaru. The three headed to meet up with the others. Mitsuri was waving at them. "You guys!" She ran and hugged them. Tanjiro explained that Mitsuri showed up as he was fighting the 'resentment' clone of Hantengu. She had kept the demon at bay until Tanjiro located the 'fear' portion inside the heart of 'resentment' and then she skillfully destroyed it. He was astonished by her blade, and described it by waving his arms around like a snake. Haku laughed at his motions. The moon was still up, but daybreak was close so the group headed indoors to receive treatment and keep Nezuko out of the sun.  Haku sighed.  she had changed fate. Nezuko would be able to be cured and Muzan did not know she had conquered the sun.