A commissioned officer of the United States Navy whose rank is above an ensign's and below a lieutenant, or a corresponding rank in other navies.
A commissioned officer in the United States Coast Guard, Public Health Service, or National Atmospheric and Oceanic Administration whose rank is above an ensign and below a lieutenant.
An assistant to another person, especially to a superior or more important person.
In literature, theatre/theater, etc., a good foil of the protagonist, a character who helps emphasize the traits of the main character.