Agamemnon - The King of Argos, the husband of Clytemnestra, and the commander of the Greek armies during the siege of Troy. Agamemnon is the older brother of Menelaus, whose wife Helen was stolen by a Trojan prince, thus igniting a decade-long war. A great warrior, he sacrificed his daughter Iphigenia in order to obtain a favorable wind to carry the Greek fleet to Troy. During the ten-year conflict, his Queen has plotted his death in order to avenge the killing of their daughter. He appears on stage only briefly, and behaves arrogantly. He goes to his death unaware of his fate.
Clytemnestra - The play's protagonist, Clytemnestra is Agamemnon's wife and has ruled Argos in his absence. She plans his murder with ruthless determination, and feels no guilt after his death; she is convinced of her own rectitude and of the justice of killing the man who killed her daughter. She is, a sympathetic character in many respects, but the righteousness of her crime is tainted by her entanglement with Aegisthus. Even so, Aeschylus makes it clear that Agamemnon's death must be avenged.
Chorus - The elder citizens of Argos, who were too old to fight in the Trojan War. They serve as advisors to Queen Clytemnestra during Agamemnon's absence, and provide commentary on the action of the play. Their speeches provide the background for the action, for they foreshadow the King's death when they describe the events of the Trojan War and discuss the dangers of human pride.
Cassandra - A Trojan priestess, captured by Agamemnon and carried to Argos as his slave and mistress. She was Apollo's lover. Apollo gave her the gift of prophecy, but when she refused to bear him a child, he punished her by making all around her disbelieve her predictions. She sees the ancestral curse afflicting Agamemnon's family, and predicts both his death and her own, as well as the vengeance brought by Orestes in the next play.
Aegisthus - Agamemnon's cousin, and Clytemnestra's lover. His father and Agamemnon's father were rivals for the throne. Agamemnon's father boiled two of his rival's children--Aegisthus' brothers--and served them to him for dinner. Since that time, Aegisthus has been in exile awaiting a chance to seek revenge for the terrible crime.
The Watchman - The man assigned to watch for the signal of Troy's fall from the roof of the palace. He is joyful at his king's return, but also is gripped with a sense of foreboding.
The Herald - He brings the Chorus news of Agamemnon's safe homecoming. An ardent patriot, he is ecstatic to see the home he thought he had left forever and provides vivid descriptions of the horrors of the war against Troy.