He used to be a kind hearted boy, a demon slayer who tried to turn his sister Nezuko back into human and kill muzan kibutsuji but in the fight against him got injected and became a new demon king. He destroyed all his previous friends which caused him a lot of suffering, so he decided to throw his previous self away so he wouldn’t suffer from guilt and misery. He is the ultimate demon. Unstoppable, powerful , immortal with hunger for humans. He is intimidating and powerful, evil and miserable.