A hero is a person "who has shown unusual bravery or courage." The dictionary says so.
We've got a lot of heroes in The Blood of Olympus, and not just because pretty much every character is the spawn of a god. Being big and strong doesn't make someone a hero (remember the cowardly lion?). It's what a person decides to do with their strength—or other magical superpower of choice—that gives them the title.
Questions About Courage
All of the characters in the book get at least one moment to show their bravery. What are the main moments for each character?
Is anyone ever scared or cowardly in the book? How do they overcome their fears and regain their courage?
Are the bad guys courageous, too? Think about Octavian or Orion, for example.
Chew on This
The Blood of Olympus is the final book in the series, so pretty much every character has to demonstrate their bravest moments when the stakes are at their highest.
The gods themselves may be all powerful, but they're not brave. They wait until the very last minute to help the demigods defeat the giants.