Colin Craven—Archibald Craven's son and Mary's cousin—is your standard jerk who hurts other people because he is hurting so badly himself. He's got a lot of reasons to feel bad about himself, though: His dad doesn't like him, everyone assumes he's too sick to walk, and he spends all of his time alone. These facts make Colin surprisingly easy to sympathize with, even when he's being a bossy bully throughout a surprising amount of the book.
If you need proof of how sympathetic the novel tries to make Colin, look no further than his first appearance in Chapter 13. Hard-hearted Mary still feels so bad for this pale, sick-looking kid that, as he falls asleep, she begins to "stroke and pat his hand and sing a very low little chanting song in Hindustandi" (13.146).
What is it about Colin inspires so much of Mary's kindness? First of all, Colin is a lot like Mary herself: He has grown up knowing that his father doesn't care much for him. And his mother is dead, just like Mary's parents. Unlike Mary's parents, Colin's mother died when he was born; it's this death (which, of course is totally not his fault) that has made it really hard for Colin's father even to look at his son. Colin's isolation and tantrum throwing reminds us a lot of Mary at the start of the book.
Colin also has it a lot worse than Mary in some ways: He has grown up totally sure that he is going to die. He also worries constantly and in secret that his back is going to grow incorrectly, making it impossible for him to function out in the larger world. Because of his supposedly fragile health, Colin has spent his life hidden from the world in his own bed; he doesn't like people to look at him, and he doesn't enjoy talking to people. So not only must Colin learn how to treat people better (like Mary did), but he also has to learn to trust his own body (unlike Mary).
As Colin learns to rely on Mary and Dickon, and as he starts working out in the Secret Garden, his physical and emotional condition get much better. But he doesn't react to this improvement in the way that Mary does.
Whereas Mary uses her new health to become a supportive friend to Colin, Colin uses his strength more ambitiously. He holds these surprisingly formal little seminars on what he calls the "Magic," the sacred life force that ties people to the larger natural world. (For more on the "Magic," check out the "Symbols" section). And he decides that, "when [he grows] up [he is] going to make great scientific discoveries" (25.36) about the workings of the natural world.
Maybe Colin's dreams for his future are so much bigger than Mary's because he is a boy and not a girl—this is 1911, after all. And maybe Colin is thinking bigger about the future because his childhood started so narrowly and with so much neglect. After all, if you've spent your first ten years isolated in bed without much support or love from your father, no one can blame you if your imagination is that much bigger and more expansive to make up for it. Whatever the reason, Colin quickly becomes leader of the little group of folks in-the-know about the Secret Garden.
Poor Colin is carrying a lot of his parents' emotional baggage. And that baggage is making him physically, as well as emotionally, sick. First off, his mother died when she fell from a branch in the Secret Garden while pregnant with Colin. Baby Colin survived, but Lilias (his mother) didn't. So he has the survivor guilt from that disaster.
And then, to make matters worse, Colin actually looks a lot like his deceased mother. Archibald, Colin's dad, can't stand how much Colin looks like his mother while definitely not being her. When Archibald comes briefly out of his grief to take note of Colin for the first time around a year after his wife's death, he sees a "small miserable looking thing." Not exactly what we like to hear as a father's response to his child.
In Colin's face, Archibald sees "the great gray eyes with black lashes around them, so like and yet so horribly unlike the happy eyes he had adored" (27.29), and with that, he starts avoiding his own son solidly from year to year.
In addition to his worries about Colin's likeness-and-unlikeness to his kind-hearted wife, Archibald also unintentionally passes on worries about his physical development to his son. That is, Colin overhears from Mrs. Medlock that his father's "crooked" (26.64) back started growing that way when he was a child. Colin becomes so sure that his back is growing "crooked" like Archibald's that he often throws tantrums to hide his fear over his own health.
So we know that a lot of psychological and emotional stuff is tied up in Colin's sickness: Colin's resentment and Archibald's grief over Lilias's death, along with Colin's horrible fear that he is going to get a "crooked" back and die. But none of these issues are actually physical.
That is, once Colin throws a huge, uncontrolled tantrum over the shape of his back, Mary gets so fed up that she feels his back for him. And it's perfectly fine. Once Mary assures Colin that, "There's not a lump as big as a pin" on his back, then he can move on from his hypochondria.
As Colin works in the Secret Garden that Lilias once loved so much, he feels closer and closer to his mother in spite of her early death. And when Archibald returns to find his son as happy and healthy as any boy anywhere, he stops resenting Colin for his almost-but-not-quite resemblance to Lilias. So everything's cured, right? Right?
Yes, everything's fine for Colin by the end of the book. But the underlying message that childhood illness can be cured through exercise and positive emotions is a little tough to take these days. That is, why should we let Archibald off the hook for loving his son only when Colin is all better at the end of the book, rather than at the beginning when Colin probably needed him more? It's horrible.
And what if Colin did have a "crooked" back or a terminal illness? Would it be somehow his fault if he kept getting sicker in spite of the Secret Garden? Positive emotions are obviously a good thing—that's why they're positive—but we understand the view that it's a little disappointing that Colin just gets better, since his illness is all in his head in the first place. That suggests that illnesses (either mental or physical) somehow should be a lot easier to get over than they often are in real life.