You had a very similar situation with Nezuko where you had become a demon and refused to even look at the sight of blood and dead human flesh.The Hashira thought it would be best to keep you around since you had potential. Out of all the Hashiras Mitsuri offered to take you into her care, she was nice so staying with her wasn’t super bad. Eventually Mitsuri had to leave for a mission and she was taking you with her. She grabbed the wooden box she used to carry you around.