Lettie is the very definition of enigmatic. On the surface she's just an eleven-year-old girl with cute freckles and a peculiarly mature outlook, but once the boy gets to know her he realizes that the surface is just that: something that obscures layers and layers of a depth that could be unfathomable:
"Once you've been around for a bit, you get to know stuff."
I kicked a stone. "By 'a bit' do you mean 'a really long time'?"
"How old are you, really?" I asked.
I thought for a bit. Then I asked, "How long have you been eleven for?"
She smiled at me. (3.64-70)
It's the smile that's the clincher. It seals the deal that Lettie knows something he doesn't—and she's not gonna tell him either. Mysterious.
But she doesn't hold things over his head in a malicious manner. If Lettie is anything she's kind, compassionate, and commonsensical—but that doesn't mean that she's about to tell him all her secrets (of which we have no doubt there are quite a few). After their dip in her ocean, when the boy gets a taste of the infinite wisdom it contains, he asks her if that's what it's like for her all the time. She merely says:
"Be boring, knowing everything. You have to give all that stuff up if you're going to muck about here."
"So you used to know everything?"
She wrinkled her nose. "Everybody did. I told you. It's nothing special, knowing how things work. And you really do have to give it all up if you want to play." (13.48)
Sounds to us like she still knows a lot more about stuff than the average Joe (or whatever our narrator's name is). And to make matters worse, by the end of the book all of the questions that swirl around Lettie and her family are still mostly unanswered. Sure we get cryptic hints at the how's and why's, but as we close the back cover we are still left scratching our heads.
This wasn't by mistake, Shmoopsters. Lettie's role in our story isn't to create a whole new mythical canon in which to believe—it's to facilitate the idea that the world is infinitely larger and more mysterious than we'll ever understand.
The things that happen in the story that stretch credibility seem much more believable if the author doesn't come up with a lengthy explanation—and having Lettie just shrug her shoulders and act like it is what it is makes us accept that there are times when the world does mysterious things, and we won't always know why. As the boy says: "I wish you'd explain properly. You talk in mysteries all the time" (10.75). And though we kind of agree with him, we also get why Lettie doesn't offer more insights than she does.
She's an eleven-year-old girl who's been eleven for a really long time, who knows things about the universe that would perplex Neil deGrasse Tyson and the Pope, and can do magical things like make an entire ocean of knowledge fit into a bucket of water. So we're just wondering: why become friends with someone like our narrator? Sure we feel bad for him and want him to come out on top, but he's kind of dull as dishwater. Where's the appeal?
It seems like the Hempstocks' typical modus operandi is to observe rather than intervene. When the opal miner kills himself they all know his motives, as well as how hard he thought about what to put in the note he left for his friends to help them understand what he's done. Instead of preventing his death, their idea of contributing to the situation is to nudge the cops to find the note, which the miner has tucked into his shirt pocket.
So perhaps Lettie's motivation comes from her relative youth (relative, that is, to her Gran and mother)—she hasn't learned how to be totally removed from the people who surround their farm. Maybe to an immortal being like Lettie, the boy is like a stray kitten that wanders into her yard, and she just wants to take care of him until he can fend for himself. Or maybe she just feels responsible for everything that happens after she allows him to come with her on her mission to bind the flea in the first place.