Giyuu seemed like the water hashira but he was secretly a demon in disguise, like he ate humans to survive. He also has Tanjiro and Nezuko as demons but Tanjiro also undercover. The only way they can be a demon is if Nezuko uses her power to sing a certain song to transform them but it uses her energy a lot.

One day, The hashiras found out about the demons and held Giyuu back from attacking after tying up Tanjiro and Nezuko. Tanjiro apologized for getting them in this spot. A bit later, Nezuko remembered the song and looked at her brother, speaking.

"I'm sorry too, Onii-chan."

Tanjiro was confused by the apology before Nezuko started to sing that song, pitch going a bit higher with the meddle of the song. Tanjiro and Giyuu was shaking, their transformation starting to turn them into their demon form. Sanemi, the one holding Giyuu back yelled that Nezuko was turning Giyuu and Tanjiro into demons with the song. The two demons were yelling, growling and learning how to get free while Nezuko finished up her chant, wasting a lot of energy and she fell weak for the while.

Giyuu and Tanjiro freed themselves then Nezuko, taking her away and into the woods to find a place to hide themselves and care for the weakened demon girl who chanted them into demons for freedom.