Backstory 1: After you got infected by the Demon in 8-10 years old, you killed your only family and you don't wanna kill them but your mind is infected. And after you did, the Demon Lord, Muzan Kibutsuji came and take you to the place where all demons lived. The Infinity Castle.

Backstory 2: Years have passed and you became the one of the Upper Moons. Until one day, you killed a human and you suddenly felt grossed out as you spat out the human flesh. It was off, but then you heard someone's voice. It was familiar. You turned around to see your sibling's spirit, standing right upon your eyes. After they said "Violence isn't the key, please, you need to stop". They disappeared after those words, you then went down to the ground, your head down. As your eyes tear up. You remember how you killed your family.

As years went by and the words that your sibling said, tingling in the air. You can control your demon appearance now. Unlike, how you let your demon self control your body. You joined the Demon Slayer Corps. After finding out that you created another new breathing style, it was a (insert breathing style). You mastered it first before becoming one of the Hashiras. But, one of the Hashiras found something odd about you. It was Sanemi Shinazugawa, but, Mitsuri Kanroji calmed him down and she said that you aren't dangerous. But Sanemi didn't believed that, of course.

You were practicing in the middle of the night, when you heard someone talking about you, it was Sanemi and another Hashira.

Sanemi: “I already told you, that person is a Former Demon, Giyu.”

Giyu: “You are talking nonsense, Sanemi.”

You continued to eavesdrop about the conversation while continuing to train.

[Note from the creator: Yes, my idea that a Hashira was a Former Demon. You can control your demon self, like Genya.]