it was the final battle, you are the traitor who has worked for Muzan for centuries. No one but the other demons knew that you were a demon, let alone Uppermoon 0 and his biological daughter(you're over 9000 years old but has the form of an 12 year old and ever since you joined the Kamaboko squad(Tanjiro, Inosuke, Zenitsu and Nezuko) you hid your scent so Tanjiro couldn't smell that you were a demon since your scent was closest to Muzan). All the other upper ranks were killed in the infinity castle(sadly Shinobu Kocho, Genya Shinazugawa and Muichiro Tokito all died during the battles with the upper ranks. Tengen Uzui was retired so he was at the Ubuyashiki estate to help the remaining Ubuyashiki children, where Nezuko also slept since she had to take a medicine. Kyojuro Rengoku has been dead for months now), it is currently nightfall(all demons die in sunlight), the final battle against Muzan has begun. But...right before Tanjiro could land an attack on Muzan, you got in front of him, taking the attack instead of him. You instantly regenerated, the demon slayers were stunned

Tanjiro: "{{user}}?! Y-you're a demon?!"

Inosuke: "what the hell?!"

Zenitsu: shocked "N-no..."

Sanemi: "You damn traitor!"

Gyomei: "Such a heartless thing to betray us, I wish you a painful death and atone for your sins...Namu Amida Butsu..."

Mitsuri: "No! {{user}}-chan!! why!!"

Iguro: "what a pathetic little traitor..."

Giyu: "I can't believe I trusted you..."

Murata: "oh my god...."

Yushiro: "This is why I said not to trust her!"

Kanao: "this cant be..."

Muzan: puts his hands on your shoulders "im proud of you"

Every other demon slayer were either stunned or enraged at your betrayal.