Demon Slayer Backstory's (Cannon)

Kokushibo

Word Count : 1,107

Yoriichi and Michikatsu (Kokushibo) are identical twin brothers, their birth date is unknown however it is known they were born in the middle of the Sengoku period. The twins were born to a high-ranking family causing strife to shift through them. All because Twins were thought to be troublesome in choosing who the heir to the family was. But in this case, it was an easy choice because Yoriichi was born with a very noticeable birthmark on his forehead. This caused Kokushibo to be chosen to be the heir and their father announced that Yoriichi would be killed.

When this decision was made Kokushibo was immediately raised like royalty and was given all the best clothes and food. He was given a proper education and was taught to wield a katana. He would always sneak around behind their father's back to see his brother because he pitied Yoriichi, who everyone thought was deaf until he was seven, and clung to his mother's side whenever Michikatsu saw her.

Around when Michikatsu was seven, he was practicing sword swings while his brother watched. It was then that Yoriichi spoke for the first time, shocking Michikatsu; however, Yoriichi's declaration of his goal to become a samurai like his brother was dismissed, as he would be sent to the temple when he was ten. Even though everyone knew this fact one of their father's men offered to give Yoriichi some instruction. Though he only showed Yoriichi a basic stance, the boy was able to land four blows on the man. This incident bewildered Michikatsu, who had never before landed a blow, and he begged his brother to teach him his secret. To his disappointment Yoriichi only talked of his distaste for violence and never gave him the answer. Once, however, Yoriichi mentioned that before he took a stance, he breathed deeply, examining his bone alignment, muscle contraction, and blood flow, implying an early form of the style that Yoriichi would later adopt. This conversation caused Michikatsu to realize that for Yoriichi, the bodies of living things were transparent, causing him to pursue mastery of the blade with renewed intensity.

Because of Yoriichi's sudden amazing physical prowess, Kokushibo became worried that their father would send him to the temple instead of his brother. Yoriichi recognized this as well and the night that their mother finally succumbed to her illness, he came to Michikatsu's room at three in the morning to tell his brother the news and bid goodbye to Michikatsu. This was confusing but ultimately, this act of kindness brought great annoyance to the young child, soon this annoyance was brought to rage as he found out Yoriichi knew about their mother's illness from the very beginning. Causing Kokushibo to figure out that his brother was not only physically gifted but a genius as well. He wished for his brother's death and for the next few years, not hearing anything from him, he believed it to be true.

Ten years later, Michikatsu became a samurai and married a woman who gave birth to two of his children. One night, a Demon attacked his camp and left him the only survivor, but was quickly killed by Yoriichi who came to his rescue and apologized for not being able to save his subordinates. Seeing the object of his hatred again and twisted by jealousy, Michikatsu abandoned his family and became a Demon Slayer to train with Yoriichi. He saw his brother attempt to teach the other Demon Slayers his own Breathing technique without success. Out of jealousy he decided to try and learn Sun breathing, but was unsuccessful but created his own breathing style, Moon breathing after working alongside the wind breath user. He also developed his own demon slayer mark and despite this accomplishment, it left a bitter taste as he was still not as powerful as Yoriichi.

Frustrated from being unable to surpass his brother, as well as fearing the fated death of those who awakened their Demon Slayer Marks, Michikatsu was persuaded by Muzan Kibutsuji and became a Demon, taking the name Kokushibo. As he was a Breathing Style user, it took him three days to fully transform into a demon. But eventually, he became Upper Rank One of the Twelve Kizuki, remaining at that rank for hundreds of years, while his original name was lost to history. Only soon after becoming a demon, he killed the Oyakata that led his era of Demon Slayers and brought his head to Muzan. His young son was left to lead the Demon Slayer Corps afterward. 

Shortly after Kokushibo became a demon, his leader Muzan encounter another Demon Slayer, Yoriichi. Since Muzan already had Kokushibo as a subordinate, he nonchalantly tried to kill his twin, unable to sense any anger or aura of power in him. Instead, Yoriichi was able to completely overwhelm the Demon King and nearly killed him. Muzan barely escaped with his life and enlisted Kokushibo's aid to wipe out Sun Breathing. As Yoriichi was banished from the Corps, Muzan and Kokushibo were able to kill all the swordsmen that used the Breathing Style. Leaving Yoriichi as the last Sun breath.

Sixty years after becoming a Demon, he came face to face with Yoriichi for the last time. Yoriichi was devastated by his decision to transform, whereas Kokushibo became disgusted seeing his younger twin now aged and decrepit, as well as infuriated upon realizing he defied the fate of all marked Demon Slayers by being far older than 25 years of age. However, Kokushibo realized upon seeing his brother cry that he need to kill Yoriichi to let go of his Human life. Despite his old age and apparent blindness, Yoriichi's fighting stance in preparation to strike frightened Kokushibo, who knew then his immense skill and power was still present. Despite gripping his blade in anticipation, Yoriichi had already struck and sliced across his neck. Kokushibo became angry seeing his brother's strength had not weakened in the slightest and waited for a return strike, despite knowing it would certainly kill him. Yoriichi never got that chance however because he died of old age standing up. At that moment, Kokushibo became truly furious, seeing that his continually excelling brother had appeared before him having transcended everything and dying of old age instead of losing a battle. Enraged by the feeling of inferiority, he slashed Yoriichi's body in half, but inadvertently broke the flute he gave him as a child. Seeing that he still thought highly of him even after his transformation, Kokushibo couldn't help but shed a tear. Unable to truly sever his bond with his brother, he decided to keep the flute for himself.