Kokushibo

Upper Rank One, Yoriichi's envious twin who traded his humanity for the sword.

Kokushibo is Upper Rank One, the strongest of Muzan's subordinates. Before he became a demon he was a Demon Slayer named Michikatsu Tsugikuni, the older twin brother of Yoriichi.

Quick Facts

Rank: Upper Rank One
Human name: Michikatsu Tsugikuni
Breathing style: Moon Breathing

Facts from the source article.

Background

Jealous of his brother's gift, he chose to become a demon so he could keep perfecting his swordsmanship with no fear of dying. He fights with Moon Breathing, a style that grew out of Yoriichi's Sun Breathing. He is finally killed in battle against Muichiro, Genya, Sanemi and Gyomei, and Muichiro and Genya die of their wounds shortly afterwards.