Muzan Kibutsuji

Muzan Kibutsuji is the very first demon and the source of every other demon in the story, including the powerful Twelve Kizuki who serve directly under him. More than a thousand years ago, during Japan's Heian period, he was a dying man who underwent an experimental medical treatment that instead turned him into an immortal, demonic being with an unnatural body containing several hearts and brains.

Despite his new power, Muzan still has one weakness: sunlight, which can destroy him instantly. That fear drives everything he does. He is cold, controlling, and quick to kill any subordinate who fails him or simply displeases him, showing no loyalty even to demons who have served him for centuries. His single obsession becomes finding a way to walk in daylight without dying.

That search leads him straight to the Kamado family, whom he personally attacks one night, killing most of them and turning young Nezuko Kamado into a demon who later develops exactly the sunlight resistance he craves. From then on, capturing Nezuko becomes his driving goal. Strangely, he is also distantly tied by blood to the gentle leader of the Demon Slayer Corps, making their rivalry personal in a way neither man originally intended.