She is portrayed by actress Hudson Leick, and in two episodes by Lucy Lawless while in Xena's body. In the rare musical episode, "The Bitter Suite", her singing voice is dubbed by the late Michelle Nicastro (who is known as the voice for Princess Odette in all three Swan Princess movies, though she didn't do her own singing in the first movie, but did in the sequels).
The Birth of a Demon
When she was still just a small girl Xena's army pillaged her village, Cirra. And amid the chaos, a fire broke out and consumed Callisto's family. Callisto not only lost her family, but her sanity and compassion. She vowed revenge, and dedicated her life to learning Xena's tactics and weaknesses to someday kill everything dear in her life. Xena changed her ways and tried to do good, repenting of her past. That did not magically undo the murders of her parents and Callisto still sought vengeance. However, she was willing to do anything to attain it. Nor did she want to be perceived as the villain and Xena the heroine so she sought to masquerade as Xena, since most people knew her only by reputation.
She starts out with mind games and treachery, in which she kills and rampages mercilessly, while telling any survivors that her name is Xena. At the end, Xena defeats her in a showdown, and then stops her from killing herself. Callisto is then brought to jail.
Callisto easily escapes later on through her great cunning. Since she cannot defeat Xena in battle, she instead decides to make her suffer by killing Gabrielle's new husband, Perdicas. Xena chases her down in a chariot race, and eventually trades blows with her. After the fight, Callisto is trapped in quicksand, and begs Xena to save her. Xena decides tolet her sink into the earth and dies.
Revenge from Beyond the Grave
After her death, Callisto was sent to Tartarus to suffer for her lifetime of evil. However, she works with Ares to invade the dreams of Xena (in Xena, the living can hear the dead when they sleep), and uses Xena's own guilt to take over her body, sending Xena to Tartarus in her place. Even Xena's closest friends, Gabrielle, Joxer, and Argo, are fooled by the ruse, until Xena/Callisto shows Joxer that she is not evil. Callisto/Xena continues to fool Gabrielle before Xena/Callisto fights her in battle.
Callisto is sent back to Tartarus, albeit in Xena's body, where she continues to suffer. Xena, in Callisto's body, restores the powers of Ares, God of War, who restores the women to their original bodies.
Immortality and Godhood
Once again in her own body, and once again suffering in Tartarus, Callisto makes a deal with Hera: one day on Earth to kill Hercules, with immortality as her prize. She poisons Hercules' family, using it to trick Hercules into leading her to the Tree of Life and giving her a Golden Apple. Hercules needed one to cure the poison, where a bite heals all ills and a whole apple grants immortality. Callisto gets one of the apples and it not only grants her immortality, but it restores her sanity as well. Sane, immortal, and very, very dangerous, she faces against Hercules and fights him. To her great chagrin, Hercules is victorious. He scars her face, re-breaks her sanity, and traps her in the Minotaur's Labyrinth.
When a cruel and dangerous Amazon named Valesca turns herself into a goddess, Xena is faced with an extremely unpleasant necessity: to ally herself with the one person she knows can defeat the Goddess, the immortal Callisto. Xena offers Callisto the power of a Goddess if she defeats Velasca, an offer which Callisto eagerly accepts. She comes to blows with the Amazon and offers an alliance between the two women, but Xena's intervention causes Velasca to think that Callisto tricked her. Velasca and Callisto later duel on a rope-bridge precariously balanced over a volcano, and while they are distracted, Xena cuts the rope, plunging the two women into eternal imprisonment in the bowls of the volcano.
She is freed from the volcano by Gabrielle's evil daughter Hope. Through a series of extraordinary circumstances, she managed to maneuver the young Hope next to Xena's son Solan, who Hope promptly killed. In a spectacular fight, Xena trapped the immortal Goddess in a cave-in, and thought her out of the way.
However, Callisto was once again freed on Xena's parent show, and followed Iolas back in time, accidentally causing the deaths of her family. She was trapped in an inter-dimensional pocket, but quickly escaped from this and allied herself once again with Gabriel's demon daughter.
Callisto makes an offer to Hope: in exchange for her protection, Hope must grant Callisto oblivion, nothingness, an end to her eternal torment. Callisto is present when Hope rises from her cocoon, a full-grown woman physically identical to Gabrielle, and keeps her word to protect her by fighting Xena. When Gabrielle sacrifices herself to keep Xena from killing her daughter, Callisto laughs gleefully, declaring that she has found a reason for living after seeing Gabrielle's pain. The revelation is short-lived, however, as a heartbroken Xena immediately stabs her with the only weapon capable of killing a God, the Hind's Blood Dagger, ending Callisto's life but not her torment, for in Xena, death is not always the end.
Angels and Demons
After being killed by the Hind's Blood Dagger, Callisto is sent to Hell, which she claims Tartarus pales in comparison to. She is given the opportunity to return to life if she can corrupt her nemesis and install Caesar as Emperor of Rome without hurting Xena. However, her blind rage causes her to fail at both tasks, and she returns to Hell as Xena is raised to Heaven following her crucifixion. Callisto attacks Heaven in an attempt to drag Xena and Gabrielle down to Hell with her, and Xena sees firsthand the suffering of Callisto. In a heroic act of kindness, Xena gives up her Light to save Callisto from damnation.
Callisto meets Gabrielle again in Heaven when the demonic Xena attacks Heaven. Through the God of Love, Callisto helps the prophet Eli bring Xena and Gabrielle back to life. Soon after, Xena becomes impregnated immaculately. Callisto later appears as an angel to Eve, the champion of Rome and the daughter of Xena, revealing to her the truth: that Callisto reincarnated herself as Xena's child, restoring to Callisto the family that Xena took from her and the child Callisto took from Xena. Eve sees the light and is redeemed, just as Callisto was before her.