* Luke Castellan: Luke is the main antagonist and is Percy's mirror image in many ways. Both are powerful demigods with a tragic past. However, Luke chose to embrace the darkness and join Kronos, while Percy fights for the good of the world. They are foils in their choices and paths, highlighting the difference between good and evil.
* Clarisse La Rue: Clarisse is a tough and assertive daughter of Ares, often clashing with Percy. She is a foil to Percy's more laid-back and heroic personality, showcasing how demigods can have vastly different approaches to their powers and responsibilities.
* Annabeth Chase: Annabeth is Percy's best friend and a strategic genius. She complements Percy's impulsive nature with her intellect and planning skills. While they work together seamlessly, their contrasting strengths and weaknesses make them a dynamic duo, highlighting the importance of different skills in a team.
It's worth noting that while each of these characters contrasts Percy in different ways, they are not simply "opposites." Instead, they act as foils by highlighting specific aspects of Percy's character and furthering the development of his journey.