Thorin, son of Thrain, son of Thror, is the leader of the company of dwarves who go on a quest to get their treasure back from the dragon Smaug (who destroyed their kingdom under the Lonely Mountain a couple of generations back). As the grandson of the (now tragically deceased) Lord under the Mountain, Thorin is a proud, kingly sort of person from the get-go.
But his pride gets him into trouble by the end of the novel. Thorin feels the "fierce and jealous love" of "beautiful things made by hands and by cunning and by magic" (1.83). And even though he has benefited from the help of many people along his way to regaining his lost treasure, including Bilbo and Bard, the actual destroyer of Smaug, Thorin refuses to allow any one else to claim to even a small portion of his recently-regained pile of gold, silver, and gems.
Even when Thorin learns that the greatest gem of his family's hoard, the Arkenstone of Thrain, is in the hands of Bard of Dale, he still doesn't really give in. Oh, sure, he promises Bilbo's portion (one-fourteenth of the treasure) in exchange for the Arkenstone to be returned to him. But, in secret, Thorin thinks that he'll be able to use his cousin Dain and Dain's reinforcements to take back the Arkenstone and keep the treasure he promised at the same time. So even though Thorin is an honorable, respectable dwarf, he's also willing to go back on his solemn promise, all in the name of gold. (For more on Thorin's Arkenstone as his idea of home, check out our section on "Symbols.")
While Thorin may be especially weak at the thought of money, he is still a brave and basically OK person. When the goblins and Wargs attack, Thorin comes hurtling out of the mountain armed to the teeth, and he fights like a true hero right into the heart of the goblin troops. And once Thorin is fatally wounded in battle, he finally realizes that he's going "where [gold and silver] is of little worth" (18.18). And so he forgives Bilbo for trying to stop a war by secretly handing over the Arkenstone to Bard. He dies with words of friendship for Bilbo on his lips.
The contradictions in Thorin's character – that he is a stubborn, proud person who nonetheless comes through in the end – seems to work with Tolkien's more general idea of dwarves as a group:
[Dwarves] are calculating folk with a great idea of the value of money; some are tricky and treacherous and pretty bad lots; some are not, but are decent enough people like Thorin and Company, if you don't expect too much. (12.6)
This description fits Thorin to a T: he thinks a lot about "the value of money," but he's also "decent enough," just as long as you don't expect him to be too generous or selfless. He is willing to help Bilbo when his burglar falls into trouble, as with the trolls, but he also wants to avoid the most dangerous parts of Bilbo's job – e.g., direct confrontation with Smaug. Thorin is also quick to anger, and he holds grudges forever (particularly against the Elvenking of the Wood-elves, who he totally refuses to talk to point-blank when Bard is trying to negotiate for some of Thorin's treasure). But, for all of that, he winds up coming through in the end when it really counts, which makes Thorin heroic enough for us.
One last trait of Thorin's that we want to mention: he is hilariously pompous and formal. He loves a good speech. Even when he's standing with twelve of his people and Bilbo on a doorstep to the Lonely Mountain with a dragon sitting and waiting for them all, he still bothers to stop and address Bilbo:
Now is the time for our esteemed Mr. Baggins, who has proved himself a good companion on our long road, and a hobbit full of courage and resource far exceeding his size, and if I may say so possessed of good luck far exceeding the usual allowance – now is the time for him to perform the service for which he was included in our Company. (12.2)
The "service" Thorin means is Bilbo's agreement to help steal the dragon treasure (he is a burglar, after all). But Thorin can't just come out and say that he wants Bilbo to go into the secret passage first. Nope, he's got to dress it up in the most formal language possible. Thorin's stiffness of manner is in keeping with his character as a proud person with intense traditions. But it also provides one of the novel's few elements of humor about Thorin, who is otherwise not so much about the laughter.