Muichiro had rough-housed the little boy Kotetsu for the key to the mechanical doll, eventually requiring it to train with.

Currently, Tanjiro and Kotetsu were watching as Muichiro fought the doll

"Wow... he's incredible... even though we are around the same age... he's talented, and a hashira."

Tanjiro muttered, him and Kotetsu watching in astonishment.