Demon Slayer: Starting with the Kingdom

(The rating has just been released and will gradually increase.)
[Female protagonist + no system + demons + Muzan's side + Yaiba the Movie: The Pillar Slayer]
Years later, when Mitsuha Tsugikuni faces the final battle against the Demon Slayer Corps in the Infinity Castle, she will recall that morning when she took Yoriichi to watch Miku Shō practice his swordsmanship.
Perhaps it was from that day on that the fate of the three siblings, Ji Guo and his sister, was already sealed...
My name is Mitsuha Tsugikuni. I am the younger sister of the Son of God, the Upper Moon One of the Twelve Demon Moons, the best secretary of the Demon King Muzan, the one who stole the power of the gods, the manipulator of fate, the only usurper in the world, and the nightmare of the Demon Slayer Corps for hundreds of years.
At the same time, he is also the one who ends the thousand-year obsession of the Ubuyashiki family.
"People die every moment in this world. Does it matter who kills whom or who is killed?"
"A brilliant resolve! Then let me see if you can break free from this prison called fate!"
"Alright, let's finish this quickly, I have to rush back for a meeting..."
P.S.: The female lead is a demon on Muzan's side, not an undercover demon who is physically in the demon camp but mentally in the Demon Slayer Corps.