Cal is Adam's and Cathy's son, Aron's brother, and the second Cain of the novel. He's also the one who makes you say aw, and who you want to give a hug from time to time throughout the book. He's a lot like Charles personality-wise (and looks-wise too, suspiciously): he craves his father's love and doesn't get it, he's misunderstood and disliked, he's cunning and clever, and he's a lone wolf. He's also afraid that he might be a little too like his evil mother.
You might have noticed that there are quite a few descriptions of Cal as dark:
Cal was growing up dark-skinned, dark-haired. He was quick and sure and secret. Adults were impressed with what seemed to them a precocious maturity, and they were a little frightened at it too. No one liked Cal very much and yet everyone was touched with fear of him and through fear with respect. (36.1.13)
Dark here works as a code for mysterious, suspicious, and arcane: no one ever knows just what is going through Cal's head. But that's because Cal is smart: he can do everything from tricking the schoolteacher into not calling on him to making a ton of money. More important, he can see the world for what it is: not only does he figure out who his mother is and what she does for a living (which apparently everyone in Salinas knows except for Cal and Aron) but the news doesn't shock him or turn his world upside down. He is a realist, that Cal.
But no one sees that side of Cal, because he's not an open book like his brother. Even when he tries to be like Aron, people seem to shun him anyway:
And what was charming in the blond ingenuousness of Aron became suspicious and unpleasant in the dark-faced, slit-eyed Cal. (38.1.1)
Poor Cal. We can't possibly understand why anyone would prefer Aron to this version of Cal, but whatever—apparently the people of Salinas prefer blondes.
Remember—Cal's biblical double is Cain, and Cain was a generally disliked guy because he was, you know, the first murderer and all. In the Bible Cain is "a fugitive and a vagabond" (Gen. 4:12), which basically means that he was shunned by people too.
On that same note, you know how Cal likes to wander the streets alone at night? Bingo. Cal "lived alone and walked alone" (38.1.5), a whole lot like the exiled Cain.
But even though he is a lonely realist, Cal has definitely got an emotional side going on; he is, after all, a teenager who is still trying to figure out the world and who he is in it. The world hasn't exactly been too kind to him either, what with the general friendlessness, the emotionally-elusive father, the goodie-two-shoes brother, and the ill-reputed mother. Can't the kid just get a break?
It's Lee who has to snap Cal out of his self-pity every now and again:
"You're pretty full of yourself. You're marveling at the tragic spectacle of Caleb Trask—Caleb the magnificent, the unique. Caleb whose suffering should have its Homer. Did you ever think of yourself as a snot-nose kid—mean sometimes, incredibly generous sometimes? Dirty in your habits, and curiously pure in your mind. Maybe you have a little more energy than most, just energy, but outside of that you're very like all the other snot-nose kids. Are you trying to attract dignity and tragedy to yourself because your mother was a whore? And if anything should have happened to your brother, will you be able to sneak for yourself the eminence of being a murderer, snot-nose?" (51.2.50)
Dang, Lee—lay off the sugar-coating a bit, will you?
There is a lot going on in this passage. First of all, you may recall that Lee isn't the first person to call Cal a "snot-nose kid"—Kate calls him that when he goes to visit her. Basically, Lee is telling Cal to grow up, to stop thinking that his is the only suffering in the world, and to not use this weird idea of being inevitably evil as an excuse to do evil things.
The buzzword here is timshel, the idea of choice. Cal has this idea that he is somehow going to turn into Kate because he understands some of what makes her a generally nasty person. Like he says to Lee,
"I hate her because I know why she went away. I know—because I've got her in me." (38.3.49)
There is definitely some resonance of Charles going on here. Remember how Kate (while she was still Cathy) doesn't trust Charles because she senses that they are similar? That's because Charles isn't a sucker like his brother; Kate couldn't play him like she did Adam. Cal is the same way: he is clever instead of naïve, and he is able to see people for what they are, which isn't always flattering.
That doesn't necessarily translate into Cal being evil, but one thing we see him talking about over and over again is his meanness. When he's still a kid and finds out about his mother being alive, he prays, "I don't want to be mean. I don't want to be lonely" (30.3.3). Kind of a weird reaction to that news, right? It's because he is afraid of using that information to hurt Aron when they get into one of their intense sibling-rivalry modes.
But what's really interesting about this moment is that Cal is acting like being mean is something that he can't help but do, and one of the big points of the novel is that Cal doesn't have to be Cain if he doesn't want to. Again we turn to the tough-love wisdom of Lee, at the moment right after when Adam has rejected Cal's gift:
"I told you once when you asked me that it was all in yourself. I told you you could control it—if you wanted."
"Control what? I don't know what you're talking about."
Lee said, "Can't you hear me? Can't I get through to you? Cal, don't you know what I'm saying?"
"I hear you, Lee. What are you saying?"
"He couldn't help it, Cal. That's his nature. It was the only way he knew. He didn't have any choice. But you have. Don't you hear me? You have a choice." (49.3.72-76)
Lee might be taking the biblical correlation a little far here (he's implicitly talking about the whole timshel thing), but the point is that Cal can't use Kate's genes as an excuse for why he is the way he is. Buck up, Cal.