assistant
A person or program that helps someone else do work.
An assistant is someone who helps another person do their work. A teacher's assistant might help grade papers, organize classroom materials, or work with small groups of students. A research assistant helps a scientist conduct experiments and record data. The president has assistants who manage schedules, answer correspondence, and help with important tasks.
An assistant doesn't usually make the final decisions, but they make it possible for others to do their best work by handling important supporting tasks.
You'll also see assistant used as an adjective, like assistant manager or assistant principal. These people have real authority and responsibilities, but they work under someone with more seniority.
In recent years, people started using virtual assistant for computer programs that respond to voice commands. These digital assistants can answer questions, set reminders, or play music, mimicking some things a human assistant might do. Whether human or digital, an assistant's purpose is to make someone else's work easier and more efficient.