Muichiro is a boy of fair skin complexion, short stature, and a lean muscular build, along with long straight hair of a raven-black color that fades to a cyan color as it approaches his waist. He wears his hair loose with two clumps protruding slightly from each side of his head, a set of marginally shorter hair left down to frame his face, and unruly bangs over his forehead. He also possesses large, empty-looking cyan eyes. He is identical in appearance to his older twin brother Yuichiro.
Following his meeting with Tanjiro Kamado, and the return of his childhood memories, Muichiro's once empty looking eyes seem to now have a hopeful glimmer that is accompanied with a more upbeat and cheery expression, reminiscent of his former demeanor before the death of Yuichiro.
Muichiro wears a black standard Demon Slayeruniform, tinted bluish black in the anime, consisting of a button-up jacket with a stand collar. However, he wears longer, looser sleeves that are more reminiscent of those of a kimono than of the standard uniform shirt-like cuff sleeves. Along with this, in the place of the standard tattsuke hakama wrapped by a pair of white kyahan around his calves, he wears his hakama in the umanori style, that along his style of sleeves, gives a loose and oversized appearance to his whole garment. He also has dark blue tabi socks and a pair of white zōri with dark blue straps.
When Muichiro was young, he wore a light, sleeveless yukata with a cyan mist design, similar to the darker one his brother wore when they were both ten.
Due to Tanjiro's presence around him, Muichiro manifested his Demon Slayer Mark during the battle against the Upper Rank 5 Gyokko. Muichiro's mark is blood red coloured, resembling mist shaped clouds on both his cheeks and the left side of his forehead.