Young Tom is the hero of our story. Well, he wants to be the hero of our story. It's up to you to decide whether or not he is one. That may be a little more difficult than you think.
When we first see him, he's daydreaming of being a hero. Almost. "He wished he could be cabin boy aboard a sky-clipper and see all the cities of the world" (1.27). This is so sad. Tom knows he can't rise above his Third-class status, so he doesn't even dream of becoming a captain, just a cabin boy. This is like dreaming of mowing the grass at the Superdome instead of actually playing in the Super Bowl.
But he soon gets his shot at becoming a hero—and impressing his idol, Valentine—when he tries to capture Hester. This plan fails spectacularly. However, Tom later gets opportunities to be heroic when he stabs and kills Grike and blows up Valentine's airship with his henchman aboard.
Here's the rub, though: Tom feels guilty about these so-called heroic deeds. He may have saved the day, but it was at the expense of another person's life. At the end of the story, Tom broods: "He didn't know what he was, but he knew he was no hero" (37.9). Maybe courage and heroism isn't all it's cracked up to be. Or we could look at it another way: Tom's guilt may make him seem less heroic, but maybe it's more heroic to feel guilty about actions that cause pain to other people. You decide.
Tom's pretty set in his beliefs when the story begins. Like most Londoners in the book, he believes that London rules and all the other towns drool. He's been raised to believe that Municipal Darwinism is the only way to live, and that Thaddeus Valentine is a hero for the people. With all of this comes the belief that anyone living differently than Londoners live is wrong.
Tom's initial jingoism is most apparent in his attitude toward non-traction towns. "[Tom] had always been taught that static settlements were dingy, squalid, backward places" (26.27). It comes as a surprise to him when he sees just how gosh-darn beautiful Batmunkh Gompa, the largest static settlement, actually is. (This is a big deal to the visually superficial Tom. You can check out the "Appearances" theme for more on this aspect of his personality.)
Tom's belief in Municipal Darwinism starts to crack when he sees that the pirate town Tunbridge Wheels treats smaller towns in the same way London treats them: badly. "London was no better than Tunbridge Wheels" (28.5), Tom realizes. London's just sneakier about it.
The cracks in the foundation of Tom's beliefs deepen when Tom sees what a tool Valentine really is. We're not just being sassy: Valentine really is Magnus Crome's tool. The Lord Mayor of London controls Valentine and uses him to do all his dirty deeds. And Tom hasn't even started to realize that he's becoming Valentine, in a way. He's doing the same things Valentine is doing—blowing up cities, killing people—just from a different side of the conflict.
Tom goes from believing in London, Municipal Darwinism, and Valentine to believing in nothing at all. All this before his sixteenth birthday. No wonder the kid is so conflicted.