mainstay; key person; central figure
Refers to a person who plays a central supporting role in a group or organization, often behind the scenes.
He always supports the team from behind the scenes as its mainstay.
あの部署の主控えは彼女だと言われている。
She is said to be the key person in that department.
Compound of 主 (しゅ, 'main') and 控え (ひかえ, 'reserve, support'). The exact historical derivation is uncertain, but the word likely emerged in organizational contexts to describe a key supporting figure.