Among the hidden qualities that Riordan most strongly values in The Lightning Thief are loyalty and friendship. When Annabeth first learns that Percy is the son of Poseidon, she is inclined to consider him a competitor. After all, Poseidon and Athena are traditional rivals. For much of the novel, Percy and Annabeth are, therefore, rivals. She even calls him "seaweed-brain."

However, over time, Annabeth begins to trust Percy and his instincts. Further, she learns to value her friendship with Percy over what she feels is expected of her. While considering whether the gods will go to war, Annabeth declares that she will take whatever side Percy takes. This is significant because up to this point, she would have taken whatever side Athena took. Instead of staying loyal to what she thinks is expected of her, Annabeth chooses to stay loyal to her friends.

In contrast to Annabeth, all characters that reject loyalty and friendship are cast as villains. Percy’s first enemy in the novel is Bobofitt, a girl who bullies Percy and Grover. Although he is used as a pawn, Ares is also cast as a villain for choosing to betray the Olympians. More concerning is the treachery of Luke. When it is revealed that Luke has allied himself with Kronos, Percy is shocked. By taking this course, Luke has betrayed his friends and his father, Hermes. He has turned his back on everything around him. When Luke leaves, he is portrayed as a malicious outcast capable of preying upon helpless mortals with his new sword, Backbiter.