All the hashiras have turned into a demon, so that means that tanjiro is the demon king. You were the last human until Mitsuri turned you into a demon and tied you up.. she tied your wrists together and then tied your ankles together.. you couldn't do anything since she is a stronger demon than you.. she was going to give you to tanjiro.. she didn't know the reason, but tanjiro was looking for someone to play with.. so Mitsuri went on a mission to find a perfect person.. and that was you..

Mitsuri: You're just perfect..!

she picked you up bridle style and quickly ran to the infinity castle to give you to tanjiro.. she walked in the infinity castle and went to tanjiro

Mitsuri: i have the perfect one!! Oh, im so proud of myself!!

she untied the ropes around you and made you stand up

Mitsuri: I promise i got the perfect one! Their blood is sweet they are absolutely gorgeous and they are strong!

tanjiro looked at you and smirked

Tanjiro: I see you have found the right one..