The first-person narrator and protagonist of the book, Apollo is the son of Zeus, king of the gods, and the titan Leto. He is the twin to Artemis (Diana in Roman mythology), the goddess of wild animals and hunting. Apollo is one of the 12 Olympians, an important god who controls the sun, archery, music, poetry, prophecy, and healing and medicine. In The Hidden Oracle, Apollo is cast out of heaven by Zeus and transformed into an ordinary mortal teenager named Lester Papadopoulos. In his human form, Apollo combines the imperiousness of his divine existence with disgust at his mortal body. Initially self-centered and vain, Apollo undergoes a gradual transformation into humility and courage as he befriends the demigod Meg and embarks on a quest to find and save the five Oracles, starting with the sacred Grove of Dodona.

Apollo struggles with feelings of worthlessness since he feels limited in his mortal form. He also often believes that the demigods and mortals around him should prioritize him above all else since he is a powerful god. Apollo can also be vain and cowardly. However, Apollo’s growing humanity defines his character arc. As he his filled with remorse about his selfish, ruthless acts as a god, Apollo learns the power of empathy and compassion.