right-hand man
noun [ C usually singular ] uk/ˌraɪt.hænd ˈmæn/ us/ˌraɪt.hænd ˈmæn/
（尤指工作上的）得力助手，左膀右臂
someone who helps and supports you the most, especially at work
How will he cope without his right-hand man? 失去了得力助手，他如何应付呢？
Compare: girl Friday man Friday