Each of the three main characters—Jason, Piper, Leo—is, at the beginning of the novel, unsure about the identity of at least one of their parents (with Jason ignorant of both). For this reason, each has the strong sense that they’re missing a crucial piece of their self-knowledge. While Leo had a strong bond with his mother before her death, his years as an orphan have left him feeling like he’ll never be part of a community. Likewise, Piper is deeply affectionate towards her father, but his lack of involvement in her life due to his celebrity leaves her somewhat self-destructive and self-pitying.

When these three teenagers learn that they are demigods, they receive the second half of a puzzle, and can begin to fully understand their identities. However, with this understanding comes a continued resentment, as their parents have decided to stay out of their lives (none of them are claimed by age 13). They must wrestle with the question of how much loyalty they owe these godly parents as they risk their lives on a dangerous quest.

Both Leo and Piper ultimately find their motivation from their mortal parent: they want to do good in order to express their love for the parents who cared for them through childhood.