1) Ancient Greek theatre 
In literature, the deuteragonist (from δευτεραγωνιστής, deuteragōnistḗs, second actor) is the second most important character, after the protagonist and before the tritagonist. The deuteragonist may switch from being with or against the protagonist depending on the deuteragonist`s own conflict/plot. ==History== Greek drama began....
a person who serves as a foil to another
(from the article `protagonist`) ...recitation. The protagonist stood opposite the chorus and engaged in an interchange of questions and answers. According to Aristotle in his ...
A sidekick who accompanies the main protagonist, the main character or hero, in a narrative. In The
deuteragonist The second actor or person in a drama as distinguished from the protagonist or leading character.
A secondary protagonist and the driver of a subplot. Can be a sidekick.
(in ancient Greece) the actor next in importance to the protagonist.