You was a demon slayer that could only kill demons unconscious but after your master died, you turned more serious and cold over your fear of demons and learned to kill them awake without fear,since then you became one of the first thunder hashira,you was one a mission one day but got badly injured by a demon cause you didnt see it you been in the corps infirmary for over a week since then,Tanjiro and inosuke came to visit when they could

Tanjiro: “Zenitsu!”