Kokushibou

Kokushibo is the secondary antagonist of Demon Slayer: Kimetsu no Yaiba. He is the strongest member of the Twelve Demon Moons, holding the position of Upper Moon One and the right-hand of Muzan Kibutsuji, and the ancestor of Muichiro Tokito (the Mist Pillar) and Yuichiro Tokito.

Before becoming a demon, he was a Demon Slayer named Michikatsu Tsugikuni and was speculated to be one of the first Breath Users. Kokushibo bears a striking resemblance to the first Breath of the Sun user. A panel in chapter 128 even raises the possibility that the two are siblings(assuming they are Kokushibo and the first Breath of the Sun user) given the striking resemblance of the two demon slayers.

Personality
Kokushibou is a mysterious but composed, grim, serious and somewhat honorable individual who never plays around. When a fight starts and his sword is drawn, he aims to bring his opponent down and break them completely. The most he'll allow is for a defeated foe to become a demon themselves.

Despite being a demon, Kokushibou relies more on his swordsmanship and Breath of the Moon techniques rather than using the supernatural Demon Blood Arts.

Muichirou is also an honorable person who gives respect to opponents he considered worthy. After discovering that Muichirou is his direct descendant and analyzing his fighting skills, he gives him great respect and even offers him to become a demon and join Muzan's army if Muzan allows it. However, he gives the greatest respect to Himejima, who he claims is the strongest Pillar he has fought in over 300 years and tries multiple times to get him to become a Demon so that he can preserve his Techniques, since the Demon Slayer Mark will kill him by the end of the night.

Trivia

 * Kokushibo is chronologically the first Demon Slayer to become a Demon.
 * Kukushibo's name means "".
 * He was a human from, at the very least, 300 years prior the current setting and with that he speaks in extremely antiquated Japanese, how much of that is translated into English varies within fan translation efforts, the official Viz Media translation puts a little more effort in adapting Kokushibou’s manner of speaking from time to time.