Atia Balba Julia Caesonia is a Roman widow. She is niece to Julius Caesar and mother to Octavia and Octavian. Atia is also the long time lover of Mark Antony.
Equal parts charming and conniving, Atia will always look for a way to gain an advantage over others. Being a woman of the Julii, she is accustomed to an easy life, surrounded by servants. She possesses an amazing knack for self preservation as evidenced by her serial avoidance of lowering status. She is also an amoral and violent sociopath who seldom hesitates to kill or torture anyone who inconveniences her.
Atia's first scene in the series sees her using her sexuality to bribe Timon, her hired hand, into giving her a white horse he intended to sell at the market. Successful, she informs her twelve-year-old son Octavian that she intends him to take the horse to Gaul as a gift to her uncle Julius Caesar. When Octavian goes missing after being kidnapped by Gauls, Atia partakes in a special ritual in which a bull is sacrificed on a platform above her, drenching her in its sacred blood; she asks the gods to protect Octavian and see him safely to Caesar.
Taking her rivalry with Servilia to a new level in The Ram has Touched the Wall, Atia pays to have graphic sexual depictions of Servilia and Caesar drawn on every street corner, humiliating Caesar's wife Calpurnia. This ends the affair, but causes Servilia to curse both Caesar and Atia.
In the episode Egeria, Atia is concerned about her son Octavian and his apparent lack of virility and masculinity; she hires Titus Pullo to train him as a fighter and to take him to a brothel for his first penetration. Upon Octavian putting on the toga virilis and observing how his fighting skills have improved, Atia suggests that he join a military academy. Though a widow, Atia is by no means resigned to celibacy and spinsterhood. She suggests to her lover Mark Antony that if they married, his social status would be elevated enough to seize control over Rome in Caesar's absence; however, Antony rejects her aggressively. Nonetheless, he later confides in Octavia that he finds himself wretched without Atia and they quickly make up their differences.
When Atia learns her daughter Octavia has seduced her brother Octavian into committing incest (under Servilia's influence) in Utica, Atia loses control and furiously confronts Octavia with a whip. When Octavian intervenes, snatching the whip from her and declaring, "I am your son, not your child. You will not strike me anymore!", Atia replies, "Will I not?" and strikes him to the floor with a blow to the face. Following this event, Octavia runs away and Octavian leaves to convince her to come back. Atia takes revenge on Servilia by having Timon and her slaves publicly humiliate her: they pull Servilia from her litter, strip and beat her in the forum and cut her hair.
When Caesar is assassinated by the Senate, Servilia invites Atia to her villa to tell her the news in person. Worried what Servilia may be planning, Atia insists her son accompany her. Atia is devastated by the news, and somewhat shaken by Servilia's vow to make Atia suffer, "Slowly and deeply, as you made me suffer." Octavian remains composed and calm, eyeing Servilia coldly.
With Octavian's input, Mark Antony manages to broker a peace between Caesar's supporters and the conspirators, ensuring the safety of himself and Atia's family. Soon Antony is welcoming Caesar's former mistress Cleopatra, Queen of Egypt, to Rome; he dismisses her request for public acknowledgement of Caesar's son Caesarion with derision. He assures a jealous Atia that Cleopatra is unappealing. Later, at a party for the Egyptians, Atia plans to have guest Servilia kidnapped and murdered; Octavian and Antony put an end to the plot as Cleopatra arrives with fanfare. Atia senses trouble from the Egyptian Queen, whom she feels is beneath her; as Cleopatra is leaving, Atia whispers in her ear, "Die screaming you pigspawn trollop." When an attempt by Servilia to poison Atia fails, Atia has her rival kidnapped, raped and tortured horribly. Finally Timon, disgusted by Atia's cruelty and overcome by an attack of conscience, sets Servilia free, throttles Atia and leaves her gasping for breath.
The growing rivalry between Mark Antony and Octavian puts them literally at war with each other; until they see the need for them to unite against the powerful armies of Brutus and Cassius. Atia goes to Mark Antony in Cisalpine Gaul to secure an alliance.
In Death Mask, both Brutus and Cassius have been killed in the Battle of Philippi, and Atia has one final encounter with Servilia. Broken and alone, Servilia curses her rival before publicly killing herself on Atia's doorstep; despite their animosity, Atia is clearly unnerved by Servilia's death as the suicide was meant to bring misfortune to Atia's house. Soon, Atia suggests that the marriage between her and Mark Antony finally occur as a show of unity between Antony and Octavian. The men agree that such an arrangement is necessary, but to Atia's surprise it is her daughter Octavia who is betrothed to Antony. Understanding that Octavia's childbearing age makes her more suitable for the match, Atia goes along with the marriage – but is furious.
In A Necessary Fiction, Atia and Mark Antony have resumed their affair, and Octavian's darker side emerges further when he discovers this betrayal (and that fact that Octavia is involved with his friend, Marcus Agrippa). He commands Antony to leave Rome indefinitely, or be publicly shamed with Octavia's adultery, and sends Atia and Octavia into seclusion (under armed guard) at Atia's villa. Antony manages a goodbye at Atia's doorstep with guards keeping them from embracing, and promises that when the time is right he will send for her. This never happens, and these are the last words he ever says to her.
In Deus Impeditio Esuritori Nullus three years have passed since the previous episode. With Rome starving from a lack of grain caused by Antony's desire to provoke Octavian into Civil War which is not ideal due to Antony's popularity. Octavian asks that Octavia and Atia go to Egypt to plead for Antony to give the grain to Rome. While Atia is ecstatic at finally being reunited with Antony she is heartbroken when Antony refuses to see her (in order not to humiliate her or kill her as Cleopatra desires). Returning to Rome while angry at her son's toying with her emotions Atia demands that he destroy Antony and Cleopatra.
In De Patre Vostro, Octavian returns to Rome and establishes his position as the first Roman Emperor. During his triumph, Atia, while making sure that she is seen as the first woman in Rome, begins to realize that she may no longer be able to control her son. She begins to sadly ponder on how Octavian developed from a good honest child to a cruel liar. When the trophies of Octavian's victory over Antony are paraded before the Senate, she is seen one last time, with a deep sadness in her eyes.
- The character of "Atia" is highly fictionalized in the TV series. In reality she had never had an affair with Marcus Antonius, and according to Suetonius she had been a pious and caring person. She does, however, bear striking similarities to Fulvia Flacca Bambula , Antony's third wife, who was known for her political ambitions and unconventional behavior.
- Despite scheming various amoral actions throughout the entire series, Atia herself is never actually seen or mentioned as killing anyone throughout the course of the entire series.