Demon Slayer: Who is Kokushibo?

gamerant.com:

Warning: This contains spoilers for the Demon Slayer manga not yet covered in the anime.

Kokushibo is a relatively recent addition to the Demon Slayer lineup and while a lot of information is known about him, he still remains an intriguing figure. BeAt the time of his introduction he was one of the oldest Demons that the audience was aware of besides Muzan himself, he has a long and confusing history full of conflicts, battles, and relationships. Frequently he was victorious, rarely did he lose, but he was never quite able to shake the worries and insecurities of his human life, even after 400 years. Ultimately his all too human failings caused his downfall, just as they caused his descent into the demon race.

Demon Slayer follows young Tanjiro and his sister Nezuko as he climbs his way up through the ranks of the Demon Slayer Corps. He does so by challenging increasingly powerful demons in an ultimate bid to defeat the leader of the demons so that he can return his sister to her humanity.

Inuyasha's Family, Explained

Kokushibo was a human during the Sengoku era, and was the older brother of Yoriichi Tsugikuni. As a human, he was a powerful swordsman and respected member of the Demon Slayers corps. However, he was never able to step out of the shadow of his twin,the inventor of Sun Breathing — from which all other breathing styles are derived. Out of a sense of inferiority, he invented his own breathing technique, Moon Breathing, though it too was derived from Sun Breathing and therefore did little to assuage his jealousy. This struggle plagued him until the moment of his death.

He studied under his younger brother and in this way was able to unlock his Demon Slayer Mark, which unlike his brother’s, was both on his forehead