Yoriichi Tsugikuni was a member of the Demon Slaying Corps during the Sengoku Era.
Yoriichi is the creator of the first and most powerful breathing technique which all other forms branch off from, sun breathing. He was also the younger twin brother of Michikatsu Tsugikuni.
He was able to defeat Muzan easily, but didn't kill him because he asked the "Demon King" a question, giving him time to escape.
Michikatsu Tsugikuni, Yoriichi's brother, jealous of his brother's abilities, accepted Muzan's proposal and became Upper Moon 1.
