Rick Riordan's The Last Olympian is the last of the five books in the Percy Jackson and the Olympians series. These are some of the themes it explores:
The topic of family runs through all the book in the series, as all demigods have to make sense of their relationships with their godly parents. Percy mentions several times that because all gods are related to each other, all demigods are one big extended family. When Luke dies, they mourn him as a brother—even though he betrayed them by joining Kronos: Percy says that his voice is breaking when he asks for "a shroud for the son of Hermes."
Demigods have to rely on each other and trust each other in order to defeat the forces that are much bigger than they are. Percy and Annabeth are loyal to each other and trust each other implicitly, even though they have very different ways of doing things. Percy discloses to Annabeth where his "Achilles' heel" is because "if you can't trust Annabeth, you can't trust anyone."
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