Zenitsu Agatsuma

Thunder Breathing has six forms. Zenitsu learned one. He tried the rest and failed at all of them, so he took the first form and did nothing but that, over and over, until nobody alive could do it better. He is also a coward. He screams, he cries, he begs not to be sent anywhere. He only fights when the fear knocks him unconscious, and then he moves faster than anyone can follow. His hearing is so sharp he can hear a heartbeat lie. He knew Tanjiro was good before Tanjiro said a word. One move. Terrified the whole time. Still went.