Thinking back to his past with Yoriichi Tsugikuni, Kokushibo remembers how he had begged for Yoriichi to die as the justice of the world went awry with his very existence. Their father had sent envoys to the temple Yoriichi was supposed to be at in hopes of making his younger son his heir but found that he had never arrived there and so Michikatsu had inherited everything.
For a decade afterwards Michikatsu lived in peace, even getting married and having a child. However, one day his military encampment had been attacked by demons who slaughtered all his troops and were about to do him in as well until a timely intervention from none other than Yoriichi.
The moment he laid eyes on his brother again, Michikatsu's tranquil days came to an end as he saw how much stronger Yoriichi had become and his jealousy and anger was only stoked by the latter's good-natured apology for not being able to save his subordinates.
Possessed by his jealousy, Michikatsu abandoned his family and joined the Demon Slayer Corps. He saw that Yoriichi had begun to teach the other Demon Slayers how to use his Sun Breathing and sword techniques. None were able to reach his level and perform them however and he had to work around this limitation, instead focusing on the strengths of each individual and teaching them a Breath Style suited for them.
More people began to grow marks like Yoriichi had and the power of the Demon Slayers increased and even Michikatsu himself grew a mark, which was the same as Yoriichi's. Michikatsu was also unable to use Sun Breathing and had to resort to using a derivative which he named Moon Breathing, anguished by the fact that he was unable to catch up to his brother. Soon the marked Demon Slayers began dying in numbers as they learned that the marks shortened their life spans, signalling that the golden age of the Demon Slayers would soon come to an end and that he himself had little time left remaining as well.
It was with these thoughts in his head that Michikatsu came face to face with Muzan Kibutsuji who offered to turn him into a demon, saying that with this nigh immortality Michikatsu would be able to perfect his techniques as much as he pleased. Michikatsu accepted the offer, becoming Kokushibo and felt as if he had been freed from all his obligations. This was fleeting however, as when he once again faced his brother Kokushibo was utterly outclassed and saw with his own eyes that his brother had transcended the world. Even Muzan had been unable to lay as much as a scratch on him and Yoriichi had died undefeated in his old age.
Out of sheer hate at once again coming up short against his brother, Kokushibo swiped at Yoriichi's standing corpse with his blade, slicing it into several parts. However, there was one final surprise still in store for him as he did so and he saw that a bag fell out of Yoriichi's robes, one that contained the flute he had made for Yoriichi when they were children. Seeing that his brother had still cherished their memories together, Kokushibo tears up but is also frustrated at his brother's continued excellence.
Kokushibo says that he had been unable to forget Yoriichi's face throughout his entire life, despite wishing desperately that he would. He said that Yoriichi had been like the sun and people around him yearned for him and to be like him but their only option had been to fail and vanish away. Recognizing that he hadn't been able to obtain anything, having abandoned everything he had to emulate his brother and had even been unable to wipe out his Breath Style despite killing all those who knew it's forms alongside Muzan. Kokushibo finally disintegrates away as he wonders why he had even been born asking Yoriichi this question as he finally faded away.
Characters in Order of Appearance
- Yoriichi Tsugikuni (flashback)
- Kokushibo (flashback)
- Muzan Kibutsuji (flashback)
- Muichiro Tokito
- Gyomei Himejima
- Tanjiro Kamado (flashback)
- Kokushibo dies.
- The origin of the different Breath Styles is explained.