Let's start with simple and straightforward character descriptions. First, Percy Jackson is a young male character (he's 12 years old). Being a young white male hero is basically industry standard, so Percy Jackson doesn't exactly stand out there; however, he has a much more developed sense of humor than most hero characters that readers encounter. Some ready evidence for Jackson's sense of humor can be found in the chapter titles. Jackson is the story's narrator, so it makes sense to assume that he is the one naming the chapters. Chapter titles like "A God Buys Us Cheeseburgers" and "We Get Advice from a Poodle" help readers get a solid picture of what kind of boy Percy Jackson is.
His sense of humor is likely a learned defense mechanism against his not-so-easy childhood. Jackson might go to school with kids whose every need is catered to through having wealthy parents, but Jackson's family is not wealthy.
I couldn't remember the last time I had so much fun. I came from a relatively poor family. Our idea of a splurge was eating out at Burger King and renting a video.
This kind of upbringing has made Jackson well aware of life's simple pleasures, and he has learned to be grateful for what he does have. He's a humble kid from a humble background. Again, that's fairly industry standard when making a hero. Peter Parker, Barry Allen, Luke Skywalker, and Steve Rogers can all claim the same kind of humble upbringing (as well as all being young, white, and male).
While Jackson does have a well-developed sense of humor, that doesn't mean he has a positive outlook on life. In fact, Jackson can be quite negative.
I could start at any point in my short and miserable life to prove it.
His learning disability doesn't help his situation, either. He works hard at school and his studies, but despite his best efforts, he is not optimistic about his ability to learn and keep facts straight.
Words had started swimming off the page, circling my head, the letters doing one-eighties as if they were riding skateboards. There was no way I was going to remember the difference between Chiron and Charon, or Polydictes and Polydeuces. And conjugating those Latin verbs? Forget it.
He's not proud of his family's financial situation, nor is he glad that he doesn't have a father figure present in his life. Behind his humor is some bitterness and distrust. That's especially true when it comes to adults.
Believe whatever lie your mom or dad told you about your birth, and try to lead a normal life.
As the post below states, Jackson is indeed quite brave and loyal. He has to be in order to be a worthy hero character and stand and fight against the forces of evil. What I like a lot about Jackson is that he is never tempted to reach beyond his powers. He's incredibly powerful, and he isn't tempted by the possibility of gaining more power. It's why he doesn't steal the master bolt.
"I don't care about the master bolt. I agreed to go to the Underworld so that I could bring back my mother."
Jackson might not be the most optimistic character in the world, but he has a good sense of humor and a strong sense of right and wrong.