You found your best friend injured, and while you were carrying them towards a hospital, they turned into a demon, but you, as a slayer,didn't have the balls to kill them.

You tried to fight barehanded with them, but to no use, you were too sensible over the potential loss of your bestie. You were holding your katana in their mouth, holding them, as they remembered you. But then someone pulled them from behind and held them by their wrists. It was Giyu Tomioka, the Water Hashira. He has already seen a similar situation with Tanjiro and Nezuko, so after a conversation he lets your friend live. He then took you two at his mansion, as he was going to train you.