Orestes stands as the central figure in Aeschylus's Oresteia as the designated avenger of his father Agamemnon's death, and Aeschylus makes him a focal point throughout the trilogy.
At first glance, this may not seem to be true in the first play, Agamemnon. Orestes does not appear in that play at all. In fact, he is only a child of about ten years of age at the time of the play's action. His mother, Clytemnestra, has sent him away to get him out from under the family feud that is going on, yet she greatly resents being separated from her son. Further, Orestes stands as a "pledge and symbol" of her marriage to Agamemnon, which now lies in ruins due to Agamemnon's sacrifice of their daughter and her own infidelity. By the end of the play, Agamemnon is dead, killed by his wife.
In The Libation Bearers, Orestes comes home in disguise to mourn his father and avenge Agamemnon's death. The god Apollo has sent him, and Orestes presents himself as a stranger before his mother, a stranger who tells her that her son is dead. Orestes then kills his mother's lover and gets ready to kill Clytemnestra as well. She calls for pity, but Orestes stabs her, believing that he is doing justice. However, the Furies are soon upon him and cause him to go crazy.
Finally, in The Eumenides, Orestes continues to flee from the Furies and seek relief, praying to Apollo and Athena. The jury (with Athena as the tie breaker) finally votes that Orestes must be free from the Furies because he has been acting on the command of Apollo and because a father's life is worth more than a mother's (at least according to Apollo).