Character Analysis
Anyone who nicknames himself the Great Brain and constantly refers to his "great brain" (no citation here, because we do mean constantly) clearly doesn't lack confidence or cleverness. And so it is with Tom, a.k.a. the Great Brain.
Let's start with the cleverness, because that's what drives him (and what drives his parents crazy). There's no question that Tom is smart. But while he is book smart, he is also incredibly clever, able to think through any problem—or any objection to his schemes—thrown at him:
Aunt Bertha shook her head. "I tell you, Tena, that boy could talk his way around anything."
"He gets it from his father," Mamma said as if she was proud of Tom instead of angry with him for marching ten kids across her clean kitchen floor. (1.51-52)
Bolstering his cleverness, as Aunt Bertha touches on here, is Tom's confidence. In fact, we might say he's downright cocky about his ability to accomplish any task he sets his mind to. Check out his after-action report on the rescue in Skeleton Cave:
"When I learned that Uncle Mark and the search party were about ready to give up the search as hopeless," he said, with about as much modesty as a plucked chicken in the window of the Deseret Meat Market, "I knew the only way to save Frank and Allan and Lady was to put my great brain to work. I would have done it sooner, but I wanted to give the grown-ups every chance. When they failed, I knew it was up to me to save the day. And now, folks, please go home. I've got to rest up my great brain so it will be ready the next time something happens which you grown-ups can't solve." (3.209)
To hear Tom tell it, if it weren't for him and his "great brain," Frank, Allan, and Lady would have been lost for good. Good thing he was around to save the day.
Not for Nothing
Despite his willingness to recognize his own value (to put it politely), Tom isn't in it simply for the glory; no, Tom is definitely money-minded, too. He likes a little something to put in his pocket in exchange for sharing his brilliance with the world. Somehow he's charming enough to avoid coming across as greedy, though he definitely is.
Importantly, we have to remember that in Adenville in 1896, a kid's only real chance of making money lies in rolling up his sleeves and getting to work—and we never see Tom do any manual labor. Neither do any of the people Tom lures into his schemes as "partners," so in this way, he's right in thinking they benefit as well. After all, we'd rather sell puppies than muck stalls an day.
It's also worth noting that though he's tricky, Tom never cheats anyone—he's scheming but fair. He can't help it if his great brain gives him so many brilliant money making ideas. Still, a key part of his journey in this book is learning to appreciate the less tangible benefits of helping people out. We know he's finally seen the light (instead of being blinded by shiny pennies) when he lets Andy Anderson keep the erector set he is owed in exchange for helping him learn to get around on his peg leg.
"Of course I want it," Tom answered, "but it just doesn't seem right getting paid for helping somebody not to be useless anymore. You keep the set, Andy. I'll come over and play with it sometimes." (8.352)
In the end, Tom proves there's a heart under that brain after all. Aw.
Tom Dennis Fitzgerald, a.k.a. The Great Brain's Timeline