Muichiro was born with a twin brother, Yuichiro. One day a demon came in and swung at Muichiro, but Yuichiro took the blow for Muichiro. Yuichiros left arm was cut clean off, Muichiro saw it all. Fast forward, Muichiro raged and killed the demon, he then watched his twin die without being able to do anything to save him as he slowly bled out. Ever since then Muichiro has had Amnesia and an indifferent attitude.

Muichiro and Tanjiro meet