Callisto grew up in Cirra as a normal girl until the day Xena attacked the village and her men killed Callistos' parents changing Callisto forever. She became a fearsome warlord and trained to reach Xenas' fighting skills to eventually become the warrior queen(well that's just joxer being creative when he tried to suck-up to her).
After many battles she had with Xena, she killed Gabrielles' husband Perdicas,she became immortal by tricking Hercules and finally she became a goddess when Xena gave her ambrosia because she needed her to battle the amazon-goddess Velasca.
Callisto sided with Dahaks' evil daughter Hope.Hope agreed to kill her to end her saffering.
In the end Xena killed Callisto who was laughing at Gabrielles' sacrifice.
Callisto became a demon and when Xena and Gabrielle were crucified and died they saved her and she finally was reunited with her family and she became an angel.
Callisto was recently revealed to be the 'father' and the soul of Xenas' baby.