kokushibo

kokushibo is the strongest demon in the 12 kizuki being upper moon 1 he is the creator of the moon breathing style he became a demon because After his subordinates were killed by Demons, he abandons his family and joins his brother to become a Demon Slayer and gains a Mark. After gaining the mark, he is told that, in exchange, he is cursed to die by the age of 25. Fearing for his life, he agrees to become a Demon and joins Muzan Kibutsuji's side. and to also surpass his brother (i havent read the Manga so im not to sure about that last part so apologies if I got it wrong)