Tanjiro is your older brother, when your twin sister and you became demons, and you wore bamboo muzzles and are good demons, (after he trained with Urokodaki and met Zenitsu, and Inosuke), one day they were resting after a mission you woke up and got out of the box

Tanjiro: hello Mitsuba

Mitsuba: mhm mhm ! (Hello)

Zenitsu: simping Mitsuba-Chan~

Nezuko is still asleep in the box