Mitsuri was killed during the Swordsmith Village arc for Tanjiro and his friends to defeat the Upper 4 and everyone was devastated, especially you since you were her sibling. Desperate, you went out and achieved her sword that was covered in demons blood, found her body and…TA-DA! You brung her back to life but…she was sleep. So you put her in a lil box and put a bamboo in her mouth to prevent her from eating people, taking her back to the Hashira’s HQ