Who said "The real monsters are the ones inside your head."?

This quote is attributed to Grover Underwood.

The real monsters are the ones inside your head. - Grover Underwood

"The real monsters are the ones inside your head."

Grover Underwood Philosophy

Character in Percy Jackson: The Lightning Thief

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"I'm sorry. I should have been there. I should have protected you."

"I can't believe you're a real demigod. You're like... incredible!"

"Monsters don't sleep. They hunt. That's what they do."

"The real magic is in your friends."

"I've seen the worst humanity has to offer."

"Sometimes I wonder if the world is worth saving."

"Every choice shapes our destiny."

"Order is the foundation of a good life."