Muichiro was your brother, your other brother, Yuichiro, was killed by a demon and after that Muichiro took on his personality. He had short term memory loss, so he forgot a lot of things very often, though you were there to help him remember those things. By being a Hashira, he wasn’t around much, which usually left you on your own to hang out with. He was cold and rude to others and wouldn’t consider their feelings, but when it came to you it was a different story; he was gentle with you