At home I made my ambitions known by parading around with a cardboard tube held to my eye, shouting, "Land ho!" (Prologue.3)
Early on, Jacob knows he wants to be an explorer. It's kind of sad when his own parents discourage him from pursuing his true identity.
[Ricky] called me Special Ed because I was in a few gifted classes, which were, technically speaking, part of our school's special-education curriculum. (1.53)
This is a quick way of letting us know that Jacob is "gifted." The story would probably pan out a lot differently if Ricky, who is less-than-gifted, were our protagonist.
"You don't strike me as a quitter."
"Then you don't know me very well," I replied. (2.68-2.69)
Jacob thinks he's a quitter because that's basically all he's tried to do—he quits his job, he quits school, and he quits leaving the house. He's remarkably successful at quitting.
I dreamed instead about my grandfather as a boy, about his first night here, a stranger in a strange land, under a strange roof, owing his life to people who spoke a strange tongue. (3.42)
Jacob is really identifying with his grandfather here because he is spending his first night on Cairnholm Island, and probably feels very similar to the way Grandpa did back then.
"We're the sickest rapping duo in Wales," Worm said. (3.73)
At least that's how the two troublemaking teens of Cairnholm want to be identified. Everyone else just thinks they're punks.
"I think—your aunt and I both thought—that there was another woman. Maybe more than one." (4.38)
If this is true, this changes Grandpa's identity in Jacob's mind from a brave explorer to simply an adulterer.
"We're peculiar," [Millard] replied, sounding a bit puzzled. "Aren't you?"
"I don't know. I don't think so."
"That's a shame." (6.17-6.19)
In this book, it's a good thing to identify as peculiar. Of course, don't all "peculiar" children just want to find others just like them?
[Emma] was heartbroken for someone else, and I was merely a stand-in for my grandfather. (8.180)
Jacob's identity as Abe's grandson isn't normally an issue, but it is with Emma because she was in a relationship with Abe. Jacob can't figure out if Emma likes him for who he is, or because he looks like his grandpa (but without the wrinkles).
[Dad] was forty-six years old and still trying to find himself. (8.216)
Jacob shows a surprising level of empathy for his father. But teenagers know better than anyone how important it is to define your identity at an early age… that way you don't end up like Jacob's dad.
"How do you know those people?"
"Because I am those people," he said. (10.216-10.217)
Dr. Golan—if that is his real name (it probably isn't)—is a master at changing identities. It helps that he's good at accents, and non-descript enough to pass for any generic old guy.