Gyomei got hit by a broom when he was younger for playing the flute too much. So to not bother anyone, he'd play the flute alone. Despite being the strongest hashira, he had a kind heart. So he would go in an empty room and play the flute. You heard the sound of a beautiful flute. It was coming from the demon slayer corp. A dangerous corp that kills demons. You followed the sound of the flute. It was the strongest hashira of them all. Gyomei. He was the one playing the flute.