Deputy
one who is an assistant or second-in-command
A deputy is a person who helps someone do their job when they are not there. "Deputy" is a rank or title. It usually refers to someone who is "second-in-command". For example: a "Deputy Master" is someone who will take over the responsibilities of the Master when the Master is not there. A "deputy conductor" will conduct an orchestra when the main (principal) conductor is not there. It is similar to an "assistant". Someone who is working as a deputy will deputize (stand in for someone else).
Sometimes other terms are used such as "vice". A vice-president will take over the responsibilities of the president when he (or she) is not there or is ill.