assistant; adjunct; lieutenant
Refers to a person in a supporting role, often in an official or organizational context. The nuance is of a trusted aide or second-in-command rather than a mere helper.
He supports the company as the president's right-hand man.
She has been appointed to serve as an assistant on the new project.
Compound of 補佐 (ほさ, 'assistance, aid') and 役 (やく, 'role, duty'). The word straightforwardly means 'assistant role' or 'person in an assisting capacity'.