Demon Slayer: Starting from the Tsugikuni Family

[Female Protagonist + No System + Evil Demon + Muzan's Faction + Pillar Slayer] Many years later, when Tsugikuni Mitsuba faces the final battle against the Demon Slayer Corps at Infinity Castle, she will recall that morning when she brought Yoriichi to watch Michikatsu practice swordsmanship. Perhaps it was from that day that the fate of the three Tsugikuni siblings was sealed... My name is Tsugikuni Mitsuba, younger sister of the Child of God, Upper Rank One of the Twelve Kizuki, best secretary of Demon King Muzan, thief of divine authority, manipulator of fate, the only usurper in this world, and the nightmare of the Demon Slayer Corps for hundreds of years. And also, the ender of the Ubuyashiki Clan's thousand-year obsession. "People die every moment in this world. Who kills them, who is killed—what difference does it make?" "Magnificent resolve! Then let me see clearly whether you can break free from this prison called fate!" "Alright, let's wrap this up quickly. I still have to rush back for a meeting later..." ps: The female protagonist is a pure demon from Muzan's faction, not a spy demon with a body in the demon camp but a heart in the Demon Slayer Corps.