Webbefore he became a demon, kokushibo in his human form was an extremely talented swordsman, however after he became a demon, not only did he have powerful blood.
As a human, he was born in an age where twins were thought to bring misfortune due to succession struggles.
Webkokushibo is the only demon that falls second to muzan, so he carries the most brute strength, as he is currently the demon that carries most of muzan’s blood.