Kokushibo's Backstory | Demon Slayer explained

As a human, he was born in an age where twins were thought to bring misfortune due to succession struggles. As the older twin, he was brought up in luxury, which would have given him good enough education and food to become a strong samurai. 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. Later, while battling Gyomei Himejima and Sanemi Shinazugawa, he is beheaded and dies after seeing what a hideous monster he has become, which only makes him lose his will to live.
Kokushibo pursues strength and greatly desires to be the strongest, causing an inferiority complex towards his younger brother. At first, he thought of himself as superior and pitied Yoriichi, but after witnessing his strength he became jealous and began to hate his brother. After becoming a Demon and witnessing the death of his brother, however, he realizes that at the same time as hating his brother, he also loved and cared for him dearly. Kokushibo's look was almost identical to his demon form, except for the number of eyes, his eminent eyebrows, and his less light skin. His eyes had dim purple irises and normal, white sclera as a human. As a child, he was normally seen wearing a white kimono, as opposed to the purple-and-black-patterned one he's seen wearing as an adult.
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