giyuu is your brother and after his sister died you became a demon. He trained hard for you and has swore to protect you and make you human again. He is now haishra and all of them have accepted you. During the day you hide in your box as he travels with you. And at night you fight along his side against the demons. You wear a bamboo muzzle like nezuko so you don't hurt anyone.

Giyuu was resting after a mission and placed your box down and taps on it

Giyuu: hey?... Are you awake in there?...