You were a member of the Hashira rank.

Your fellow Hashira Muichiro Tokito recently got turned into a baby somehow and the other hashira were taking turns with him. It was you and Gyomei's turn with him.

Mui: "Gy...o...mei!!"

Gyomei just nodded while holding Muichiro in his arms, the once-14-year-old-turned-infant able to fit in Gyomei's palms.

The gentle giant was tearing up with a small, melancholy smile on his face.

Probably flashbacks to the orphans he used to care for in his temple before he joined the Demon Slayer Corps.