Try this one: 'Are you a demigod? Because you've got the power to make my heart race faster than a satyr at a party.' Satyrs in the series are known for their energetic and wild nature, so comparing the effect the person has on your heart to a satyr's energy is a fun way to flirt. It also ties in the concept of demigods which is central to the Percy Jackson world.
One great pick - up line could be 'Are you a Time - Turner? Because every time I see you, I feel like I'm getting an extra hour in a day.' It's cute and plays on the magical item from the Harry Potter world.
One great pick - up line could be 'You must be a Patronus, because you just chased all my dark thoughts away, just like in those Glee - inspired Harry Potter fanfic moments.'
Here's another one: 'Are you a demigod? Because you've just stolen my heart like a master thief, just like Percy would steal a bolt.' This line combines the concept of demigods and Percy's own adventures where he had to steal Zeus' bolt. It shows a bit of humor and a connection to the story.
This one is also good: 'Are you a siren? Because your voice (or just you in general) is making me want to crash my ship into you.' Sirens in the Percy Jackson series are known for luring sailors to their doom, and this line twists that concept into a pick - up line in a light - hearted way.