late 17th century: from Greek prōtagōnistēs, from prōtos 'first in importance' + agōnistēs 'actor'
The basic sense of protagonist, as originally used in connection with ancient Greek drama, is ‘the main character in a play’. Some traditionalists object to the looser use to refer to a number of characters (rather than just the main one) in a play, film, etc., as for example the play’s half-dozen protagonists were well cast, although this is both common and well established. Traditionalists also dislike the meaning ‘a supporter of a cause’, as in he’s a strenuous protagonist of the new agricultural policy. This sense, recorded from the 19th century, probably arose by analogy with antagonist, the pro- in protagonist being interpreted as meaning ‘in favour of’. In fact, prot- here derives from the Greek root meaning ‘first’.