What we love best about Josh is that he's a totally normal teen:
He was a normal high school sophomore, not too brilliant, but not stupid either. He played football, sang—badly—in his friend's band, had a few girls he was interested in, but no real girlfriend yet. He played the occasional computer game […] He loved The Simpsons and could quote chunks of episodes by heart, really liked Shrek, though he'd never admit it, thought the new Batman was all right, and that X-Men was excellent. He even liked the new Superman, despite what other people said. Josh was ordinary. (5.13)
See, folks? Teen magicians: they're just like the rest of us! They like so-so movies and crush on their classmates. We feel connected to Josh after finding out that he's just like us; after this introduction, we just want to know him even more.
Of course, we know that he's not just any teenager—he has magical powers within him that can either save or destroy the entire world. But we'll get to that later. The point is, in the beginning, when we're getting to know Josh, he's an everyman. And the fact that he's an everyman means we know just how he feels when he finds himself in the middle of an epic fight over the fate of civilization. As our narrator says: "ordinary teens did not find themselves in the middle of a battle between two incredibly ancient magicians" (5.14).
Although Josh is one of the heroes of our book, he definitely has his flaws—he nearly gets Flamel, Scatty, his sister, and himself killed when he accidentally insults Hekate at dinner, and Flamel warns him seriously that he will not survive unless he stops making insensitive comments about things he does not understand (such as when he calls the wereboars' faces "piggy").
Then, when Josh tries to reach out and touch Scatty, she throws him to the ground, asking his sister Sophie if Josh always acts like this:
"Like what?" Sophie asked.
"Foolish, ill-advised, reckless... Shall I go on?"
"No need. And yes, he's usually like this. Sometimes worse." (22.5-8)
Josh is definitely an agent—a character who acts on his world rather than just reacts to it. While some characters just sit back and enjoy the show, for better or for worse, Josh makes stuff happen, even if sometimes he does so without thinking first.
Sometimes his impulsiveness stirs up trouble, but sometimes it saves the day, like when he runs the Hummer into Dee in Ojai. Of course we have to remember that characters like Scatty, who have been around for a while, might be looking at Josh and seeing a stereotype. After all, teens are often called impulsive. It's just that in Josh's case, the shoe fits.
Josh loves his sister Sophie, sure, but it can't be easy for him to cope with her new powers. When she is Awakened by Hekate, Josh is left in the magical dust, and this changes their relationship more than a little: "Josh knew deep down that he had also lost his twin, the constant in his life, the one person he could always count on. […] Even when Sophie's newly Awakened senses were making her sick, he was jealous of her abilities" (37. 27).
The twins were once inseparable, but now that Sophie has got some mad skills, and Josh is still plain old Josh, they're not so joined at the hip. In fact, as we near the end of the novel, we often find Josh alone, as he is when Dee spots him by the fountain. His loner status makes him an easy target for Dee's manipulation; the guy knows Josh is looking for a new idol, and he thinks he can fill that role.
See, we know for a fact that Josh used to look up to Flamel as "the older brother he always wished he had" (30.18), but once he feels betrayed by the Alchemyst for putting Sophie in danger, he is left bitter and vulnerable. Flamel just completely changed Josh's life, without so much as a please or a thank you, so Josh is understandably a bit, well, ticked off.
But a ticked off teen is an easy target for a smart, capable villain. When Dee sees Josh pouting by the fountain in Ojai, he sees a golden opportunity and tries to tempt the teen to come over to his side of the fight. And it is a tempting offer.
But Josh's inner strength proves to be stronger than Dee's honeyed words. At the end of the day, no matter how much of a stranger his twin has become, Sophie is still his sister, and Josh is still an awesome, loving brother:
What could he do against this powerful magician? He wasn't like his twin: he had no powers. But that didn't mean he was useless […] the necromancer managed to fling himself to one side before the black Hummer crashed into the wall, pounding it to dust […] The door opened and Josh climbed out and carefully lowered himself to the ground. (39.7-24)
See, Josh knows in this moment he needs to protect his sister. It's that bond that allows our somewhat impulsive, somewhat bitter Josh to rally and save the day.
This problem crops up again, when once more, Josh finds himself left behind. When the others jump through the mirror in Dora's shop and head to Paris, Josh is a little late to the game. And Dee spots another opportunity.
But the love between Josh and Sophie proves to be his saving grace. When Sophie comes back through the mirror for her brother, we know that their sibling bond is more powerful than the temptation to hitch his wagon to Dee's rather dubious star. But the scene has planted a seed of doubt when it comes to Josh's future fate. Will he stay Team Flamel? Or will Dee make him an offer he can't refuse?