Eteocles
The son of Oedipus, previous King of Thebes, and brother to Polynices, Antigone, and Ismene. He has a curse laid upon him and his brother Polynices by their father, Oedipus, which prevents them from peacefully settling their disagreement about who should be King of Thebes. Thus, at the beginning of the play, Eteocles is preparing Thebes against the incoming Argive invasion led by his brother Polynices. Throughout the play, he gives orders to the messenger, mostly detailing which Theban champion will meet each Argive champion that appears in the seven city gates.
When the messenger brings the name of the last and seventh "Champion" to attack the seventh gate, it turns out to be Eteocles' brother Polynices. He ends up choosing himself to go fight and defend the seventh gate since there is no one else left to do so. At this point, he dies. The herald later informs us that for his honorable service, he is to be given a proper civil burial, in accordance with the honors befitting of a King. Though Eteocles dies, the invasion if defeated and the "Seven Against Thebes" are also destroyed.
Messenger
The unnamed messenger is the character who relays to Eteocles the states of affairs on the battlefield, particularly which enemy Argive champion is attacking each gate. He describes each warrior, particularly their shield, their proficiency in combat, and the god(s) that they pay homage to. He then dispenses Eteocles order for the responding champion back out to the battlefield.
Chorus
Greek plays are traditionally restricted to only one character on the stage at the time. Thus, the "Chorus" is who the main character on the stage typically converses with. They offer commentary, analyses, insight, and even debate on to the actions being undertaken. They are a guide to help the audience follow the action. Typically a group of fifty divided into two groups, they sing in unison in odes, which is divided into the strophe, antistrophe, and finally the epode. Aeschylus is credited with first having more than one actor on the stage at a time (typically at max two characters), but the chorus is nevertheless an integral part of his tragedies.
Antigone
Sister to Eteocles and Polynices, and thus daughter of Oedipus, she appears at the tail end of the play. When the herald announces the new decree after the battle that Polynices body will be left to rot, she refuses, and states she will bury him regardless of the consequences. This sets the stage for another tragedy, Antigone by Sophocles, though this ending scene is typically regarded as a later interpolation and not a part of Aeschylus' original tragedy.
Ismene
Sister to Antigone and also Eteocles and Polynices, she appears with her sister Antigone at the end of the play. While she is addressed, she has no speaking lines in the play.
Herald
Once the messenger announces Eteocles and Polynices death at the seventh gate, the Herald appears to announce the new King. He also announces the new decree(s) of the king, which mainly states that Eteocles will receive an honorable burial while Polynices will not. He then argues with Antigone, stressing to her the penalty for disobeying the new King's order.
Seven Against Thebes
The seven individuals who stand against Thebes and attempt to overtake it with their army from Argos. All seven champions are defeated, with the seventh being Eteocles' own brother, Polynices.
