Percy is a kid who is diagnosed with ADHD (very common among demigods and it helps them concentrate on everything during a battle). Mainly he’s very energetic, obvious, loyal, brave and caring. He’s also known with his usually-crazy ideas that seem to work out.

The series mention his loyalty as his fatal flaw. He is a guy who would sacrifice the world if it means saving his loved ones. That also means he’s more than ready to sacrifice himself if necessary (which he’d almost done several times).