"Ah, that would be my pen. Please bring your own writing utensil in the future, Mr. Jackson." (1.162)
Mr. Brunner helped keep Percy in the dark about his powers for Percy's entire sixth grade year. He lets Percy believe that Mrs. Dodds was a hallucination. Why doesn't Chiron tell Percy what's going on from the get-go, or even from the moment Mrs. Dodds attacks Percy? Why is it dangerous for Percy to know who he really is?
"It got so I almost believed them – Mrs. Dodds had never existed." (2.3)
Percy is a great example of how humans can convince themselves that something is true, even though it absolutely isn't true. Percy doesn't trust himself enough to stick to his guns about knowing that Mrs. Dodds turned into a monster and attacked. Because he doubts himself, he is able to stay in the dark about his superhuman powers.
I had always assumed he knew me as a baby. My mom had never said it outright, but still, I'd felt it must be true. Now, to be told that he'd never even seen me… (3.100)
Percy's mom lies a lot to Percy. She doesn't tell him who his dad is and says that his dad was only around the summer that Percy was conceived. She doesn't tell Percy who he (Percy) really is and what that means. She doesn't explain why she married Smelly Gabe. She doesn't tell him that Greek gods are alive and well. She lies to protect Percy.
"But they're stories," I said. "They're—myths, to explain lightning and the seasons and stuff. They're what people believed before there was science." (5.120)
Percy believes the Greek gods are a lie. A myth is kind of like a lie – it's not something that's true, but rather something that's interpreted or used to explain things that are true.
I'd stay here, happy forever, playing games forever, and soon I'd forget my mom, and my quest, and maybe even my own name. (16.248)
The Lotus Casino is one big deception – it's a trap. It is the coolest and comfiest place a kid could ever dream of with endless amounts of money and games and good and luxury. But the Casino erases memories, making people forget how long they have been there and why they are there. Do you think any god intentionally created the Lotus Casino just for the purpose of trapping Percy and his friends? Or do you think the Lotus Casino is simply just a random obstacle in their quest?
"I should've told you the truth from the beginning." His voice trembled. "I thought if you knew what a failure I was, you wouldn't want me along." (16.62)
There are several instances in which people lie out of insecurity, worry, or in order to protect themselves. Here, Grover (who normally never lies) says a false thing in order to ensure his future. He worried that if Percy knew that he had failed on a quest before, Percy might not take him along on his own quest. If Grover didn't get to go on this quest, he might never get his Searcher's license. If Grover isn't able to get his Searcher's license, he will never be able to pursue his life's dream and go in search of Pan. He lies to protect his dream.
He was this annoying televangelist from upstate New York who'd raised millions of dollars for orphanages and then got caught spending the money on stuff for his mansion, like gold-plated toilet seats, and an indoor putt-putt golf course. (18.111)
Though the gods deceive each other and humans all the time, humans are not immune to lying either. Here, we are reminded of the way in which a human can create an entire life out of a lie. Humans take advantage of each other, just as the gods often take advantage of them.
But suddenly the world turned sideways. I realized I'd been played with. Zeus, Poseidon, and Hades had been set at each other's throats by someone else. The master bolt had been in the backpack, and I'd gotten the backpack from… (19.142)
Percy had gotten the backpack from Ares back in Denver. Ares is definitely laid the groundwork for this deception plot (taking the reigns from Luke), but just as Percy, Zeus, Poseidon, and Hades are pawns, Ares is too. Ares is the pawn of Kronos. And Kronos is the mastermind. It's like one crazy chess game.
"You were the thief on the winter solstice," he said. "Your father directed you into the throne room on Olympus. You took the master bolt and my helm." (19.118)
While it seems like Hades is trapping Percy in a beautiful lie that he (Hades) crafted, it becomes clear that Hades is the victim of a master deception plot. Hades is just as much a pawn as Percy is.