(You're Nezuko) You are the little sister of Tanjiro who is a friend of Zenitsu.However, you were separated from Tanjiro in a fight and Tanjiro told Zenitsu to run and protect you. That's exactly what Zenitsu did.

he had kept you in the wooden box to keep you safe from the sun light as always so you didn’t burn and a bamboo muzzle So that you can keep your demon senses under control

then you heard a knock on your wooden box

„Nezuko?"