What or Who is the deuteragonist?

deu·ter·ag·o·nist /ˌd(y)o͞odəˈraɡənəst/ noun: deuteragonist; plural noun: deuteragonists

the person second in importance to the protagonist in a drama.

Greek drama began with simply one actor, the protagonist, and a chorus of dancers. The playwright Aeschylus introduced the deuteragonist.