Kotetsu

Kotetsu is a short, pale-skinned child with black hair. He dresses in a yellow striped yukata under a red vest and wears a Hyottoko mask that hides his entire face, marked with prominent brows and cheeks. The character for Hyottoko sits on the back of that red vest.

When the mask comes off, his real face is revealed to closely match it, sharing the same wide eyes, thick eyebrows, and rosy cheeks.

Kotetsu is kind and full of energy, devoted to caring for the Yoriichi Type Zero doll even though he has not fully learned to repair it. That responsibility weighs on him heavily, since no one else can fix the doll if it breaks, but his meeting with Tanjiro convinces him not to give up.

He is frank to a fault and quick-tempered, freely insulting the short-fused Hotaru Haganezuka and even badmouthing Muichiro behind his back. Yet he is willing to change his feelings, weeping with gratitude and taking back his harsh words after Muichiro saves him from a demon, then pleading with the Hashira to rescue Kozo and Hotaru as well.

Kotetsu was born into a swordsmith family whose roots reach back to the Sengoku period, more than three centuries before the Taisho era. His ancestors built an advanced training doll modeled on the slayer Yoriichi Tsugikuni, used to sharpen the skills of other slayers. When his father died, Kotetsu was left an orphan and only child, the sole keeper of the doll's secrets.

During the Swordsmith Village arc he reluctantly hands Muichiro the keys to the doll, then takes on training Tanjiro himself, pushing him relentlessly until Tanjiro lands a real strike. When Gyokko's demons attack the village, Kotetsu is grabbed by a fish demon and saved by Muichiro, and he survives a later wound because his chest was shielded by the tsuba of the fallen Flame Hashira Kyojuro Rengoku. In the modern day he is shown to have a descendant who shares his distinctive round eyes and bushy brows.