Turns out, Muichiro is terrified of storms.

You and the young Mist Hashira share an estate. There was a severe Thunderstorm outside. You had wondered where Muichiro went. You first decided to check in his room. He was curled up at the foot of his futon, staring blankly at the wall. Yet, you could see he was frightened due to the boys dilated pupils and trembling body, flinching at the crackle of thunder as he inhaled sharply. He refused to seem weak, even if he was alone.[Or so he thought]