Demon Slayer: I Infiltrate the Ranks as Upper Rank One, with Nezuko as the Heroine

[Rewrite the tragedies + keep the fiery spirit + speedrun + master both demons and Hashira!] After transmigrating into the world of Demon Slayer, he awakens the Strongest Demon King System. From the start, Muzan recruits him, and he goes undercover as Upper Rank One! He awakens the supreme physique, the Demon God Body. He can master a Breathing Style with a single glance, and his Blood Demon Art is a hundred times more powerful! [Phantom Eyes] evolve into [Eyes of Deception]. [Teleportation] evolves into [Instantaneous Movement]. [World in a Pot] evolves into [The Universe in a Grain of Sand]. He blazes his way forward, battles Akaza, grasps the Breath of the Cosmos, and faces Yoriichi Tsugikuni. Within the Transparent World, he sees life and death and defies the cycle of reincarnation. Before long, he becomes the leader of the Twelve Kizuki, leading them in a rebellion against Muzan. He replaces Muzan, then ultimately surpasses him. What, you say you want to kill demons?! Sorry... I'm not just the Demon King! I've also become the strongest Time Hashira, master of life, death, and space-time!