Mechamaru was quite a minor character in Jujutsu Kaisen, not being known to fans due to attending the Kyoto prefecture Jujutsu High rather than Tokyo High with Itadori and his peers. He was briefly introduced during the Goodwill Exchange Event, with the breadth of his abilities barely being shown. However, in the anime's recently aired second season, Kokichi Muta got more than his moment in the spotlight. Mechamaru was a vessel Kokichi used to attend school as, due to his Heavenly Restriction, he was too frail even to see the light of day.
Source：https://www.cbr.com/jjk-machamaru-special-grade/
