The character you might be thinking of is Annabeth Chase, who, in *The Sea of Monsters*, initially believes Luke betrayed Percy based on misleading information. However, she soon realizes this is not the case and that Luke was actually trying to protect Percy.
While Luke does turn against the gods and become the main antagonist in the *Percy Jackson and the Olympians* series, he does not betray Percy in the traditional sense. He still has feelings for Annabeth and cares about Percy's well-being.
It's important to distinguish between betrayal and opposing forces. Luke makes a choice to oppose the gods, but he doesn't betray Percy personally.