A person who provides services to those engaged in a particular endeavor.
A civilian who follows an army, typically a family member of a soldier or someone who provides services to the soldiers, e.g. prostitution, sale of liquor, cooking and laundry.
One who joins a group, movement, or project without being a legitimate member; one who jumps on a bandwagon.
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.