Calliope is the mortal daughter of Kratos. When the great Spartan captain fell into the service of Ares he became ruthless as he carried out the God's evil and brutal skirmishes againt the people of the land. Kratos was ordered to pillage and destroy one particular small village. He charged with his horde of soldiers and tore the villagers limb from limb, some tried to escape into the local chapel but Kratos spotted them and chased them inside where alone, he slew them all in a fit of burning rage but as he came back into focus, the smouldering timbers searing away at his flesh, he recognised the faces of his wife and only daughter, Calliope. It was then that Kratos was imbued with such anger that he was reborn as the ghost of Sparta.
There are many few things Kratos cares about in his life. He cares not for wealth, harmony or trickery but favours those closest to him and his own personal gain (revenge). Calliope and her mother were the only two things Kratos had left in his life and would stop at nothing to get them back. In Chains of Olympus Kratos encounters Calliope in the underworld where they reunite and Kratos vows never to leave her. But as his life and magic energy started to sap from him as he became closer to his daughter he had to stop and instead turned to fight Persephone and the titan, Atlas.