second in command 
noun
The officer next in authority to the commanding or chief officer.He and his second in command then abandoned their army, leaving Grant's friend Buckner to surrender to him....
- It is difficult to see Robinson as anything other than a second in command, a loyal lieutenant who fulfils his master's wishes.
- I was still looking shocked as I dialed a phone number, not Stallings, but his second in command.
Synonyms
deputy, number two, subordinate, assistant, right-hand man/woman, wingman, vice-, understudy, lieutenant, adjutant, subaltern
informal sidekick