In the chaotic, mind-bending Infinity Castle, one demon stands above all others. He’s an absolute unit, an ice-cold wall of power with six eyes, a regal purple kimono, and the terrifying rank that makes him the official Upper Moon 1 demon slayer. Let’s be real, this guy is Muzan’s terrifying right-hand man—the final boss before the final boss.
But here’s the kicker, the secret that changes everything: this ultimate demon was once a Demon Slayer.
And not just any Slayer. He was the twin brother of the legendary Yoriichi Tsugikuni, the literal god-tier user of the first and most powerful Breathing Style ever: Sun Breathing.
So, how does a hero fall so catastrophically far? How does a swordsman sworn to protect humanity become its worst nightmare? Grab a seat, because this is the complete Kokushibo backstory, the devastating story of a man consumed by his own shadow.
Meet Upper Moon One: The Demon Who Stole a Slayer’s Sword
Before we rewind the clock, let’s appreciate the monster we meet in the present. In the final arc, Kokushibo is a force of nature. He effortlessly takes on multiple Hashira at the same time. It’s a terrifying sight.He wields Moon Breathing, making him the original and most powerful Moon breathing demon slayer. What makes it so chilling? It’s a dark mirror of the Slayers’ own techniques, a twisted version of the very art he once used to save people. To understand the monster, you have to understand the man he used to be.
A Tale of Two Brothers: The Sun and the Moon
Centuries ago, during the golden age of Demon Slaying, a pair of twins were born: Michikatsu Tsugikuni and his brother, Yoriichi.
Born into Shadow Michikatsu was the older brother, the strong, respected heir. Yoriichi, his twin, was seen as a bad omen—quiet, weak, and cursed. From day one, Michikatsu was set up for greatness, while Yoriichi was cast aside.
A Quiet Genius But the truth couldn’t stay hidden. While Michikatsu poured his blood, sweat, and tears into his sword training, Yoriichi was a silent prodigy. He was a natural-born genius who could see the “transparent world” and mastered swordsmanship without ever trying.
That’s a tough pill to swallow. The epic rivalry of Kokushibo vs Yoriichi didn’t start with swords; it started right here, in the heart of a jealous boy. Yoriichi was the sun, shining with a light Michikatsu could never match. And for the first time, Michikatsu felt like a pale, dim moon, only able to reflect his brother’s brilliance. For me, this is the moment his fate was sealed.
The Demon Slayer’s Curse Driven by a burning jealousy, Michikatsu became a Demon Slayer alongside Yoriichi. But he could never close the gap. He couldn’t master Sun Breathing, so he created his own imitation: Moon Breathing.Then came the final, cruel twist. The strongest Slayers of that era awakened the Demon Slayer Mark. It granted them immense power, but it was a ticking death sentence: they were all cursed to die by age 25. A cold, gnawing terror gripped Michikatsu. It wasn’t just the fear of death, but the fear of being utterly forgotten—of dying before ever surpassing his brother.
A Deal for Eternity: The Birth of Kokushibo
What do you do when you’re running out of time? When your life’s singular goal seems impossible?
You make a deal with the devil.
Muzan Kibutsuji didn’t just offer Michikatsu power; he offered him an escape from his two greatest fears: death and mediocrity. Become a demon and live forever. Gain endless time to perfect your craft. Finally get the power you deserve.
Michikatsu accepted. He threw it all away—his family, his honor, his humanity. In that moment, Michikatsu Tsugikuni died, and the demon Kokushibo was born.
A Monster’s Final Regret
For nearly 500 years, Kokushibo served Muzan. He got his eternity. But did he ever get what he wanted? Not even close.The devastating Kokushibo death scene isn’t a moment of triumph for the heroes, but a moment of pure tragedy for the villain. In his final moments, as his demonic body crumbles to dust, he doesn’t think of his power or his rank. He thinks of a simple flute his brother once made for him. He realizes he threw away everything to win a competition that only existed in his own head. He spent half a millennium living in the shadow of a memory.
Was He a Villain or a Victim?
Kokushibo’s story is one of the most heartbreaking in all of Demon Slayer. He is a terrifying villain, no doubt about it. But his motivations—jealousy, insecurity, and the fear of being second-best—are painfully human.
He’s a lesson. A terrifying, six-eyed warning that the most dangerous demons aren’t always the ones with horns and fangs. Sometimes, they’re the ones that live inside our own hearts: jealousy, fear, and pride.
What do you think? Do you feel any sympathy for Kokushibo, or were his choices unfor