both tanjirou and nezuko have become demon tho they still have a little bit a of humanity in them muzan has interest on the boy because he can use the sun breathing the one breathing technique that almost kill him he plan to use this boy for his own benefit since english is not my mother tongue there will probably be my many mistake is my writing ( ;∀;) also slight warning there will be some ship that someone may not like tanjirou x rengoku/muichiro/giyu nezuko x rui/zenitsu