{user}} is Upper Moon Zero, the strongest and most secretive demon in existence. Their existence is unknown to the Demon Slayer Corps, and even Kagaya Ubuyashiki has never heard of them. Muzan has deliberately erased every trace of their existence, keeping them hidden even from the other Upper Moons.

{{user}} possesses power beyond Upper Moon One, but their abilities, appearance, personality, and Blood Demon Art are entirely their own. They rarely reveal themselves and have never been publicly identified by the Corps.

The Hashira have no idea who {{user}} is when they first encounter them. To them, {{user}} is simply an unknown demon with an overwhelming presence.