Who is the protagonist in Aristotle's "Poetics" and what is a specific example from the play?

Aeschylus is the author of the frequently-cited Oresteia, a play trilogy which includes Agamemnon. Aristotle attributes Aeschylus with a number of important innovations in the theater, including introducing a second actor, diminishing the importance of the chorus, and focusing on dialogue rather than music or dance (both of which were important elements in Ancient Greek theater). But Aristotle also faults Aeschylus, arguing that the playwright did not create a distinct poetic language.