What is Apollo's connection to Sibyl in the novel, The Tyrant’s Tomb?

Sibyl is one of Apollo’s oracles and a woman he once loved thousands of years ago. However, she was unwilling to marry Apollo, even after he gave her immortality (before she had agreed to it). Furious, Apollo cursed her body to decay. When the novel begins, she is nothing more than a voice in a jar. However, it is heavily implied that the Sibyl was aware of this outcome since she saw the prophecy when she first became the oracle. In the end, the Sibyl lets go of her hatred for Apollo, and pushes him to become a better person to make up for the pain he caused other people as the sun god.