Thorin was the leader of the dwarven party in "The Hobbit" that recruited the Hobbit Bilbo Baggins as their fourteenth member since they believed to be only thirteen members would bring them big misfortune - it seems that even dwarves were superstitious.
Thorin was the son of Thrain and grandson of Thror, last known "King under the Mountain", who had his realm under Mount Erebor - the Lonely Mountain - east of Mirkwood and south of the Iron Hills just beyond Lake Esgaroth. All three were direct decendants of Durin the First, one of the dwarven ancestors made by Aule at the very beginning of Arda.
We don't know exactly how old Thorin was at the time of the dwarves' expedition to Mount Erebor, but we can establish with certainty that he was more than one-hundred years old, since Gandalf says in chapter 1 of "The Hobbit: "And Thrain your father went away on April 21st, a hundred years ago" . Earlier in the chapter Thorin mentions that he was outside the halls with another group of dwarves when the dragon attacked and states "A fine adventurous lad I was then". We also know that his nephews Kili and Fili were at least fifty years younger than him and that Thorin was not the oldest of the party, so a rough guess and speculation on my behalf would be between one hundred twenty and one hundred fifty years of age.
He set up this party in order to reclaim the old halls in Mount Erebor and drive out the Dragon if they could. So we see he was courageous and purpose driven. But the story makes it clear that he was arrogant, quick to anger, mistrusting, and an able fighter. On the more positive side we can say about Thorin that he is fond of old stories, songs and plays an instrument - a harp (a golden harp wrapped in green cloth) - and that he redeems himself at the end, as we shall see.
Along with the other Dwarves Thorin was first captured by the three Trolls Bill, Bert, and Tom not too far from Rivendell, then captured by the goblins and spiders. The whole party was rescued by the Eagles, and after a rest at the wondrous house of Beorn went on through Mirkwood only to be captured again by the Wood-elves. How they were rescued every time is told elsewhere but they finally made it to Erebor and upon setting his eyes onto the legendary crystal known as the Arkenstone he became obsessed by it and closed himself off and set his party in opposition to the Men of Esgaroth City and the Wood-elves in the brewing Battle of the Five Armies.
The Goblins and Wargs from the Misty Mountains and Gundabad were also marching towards the Lonely Mountain but luckily the Eagles of the Misty Mountains -alerted by Gandalf and possibly Radagast - were on their way too and as battle was finally joined the Dwarves (Thorin in the meantime had reinforcements led by his brother Dain II of the Blue Mountains) merged (reluctantly and somewhat surpisingly) with Elves, Men and Eagles.
Thorin was wounded mortally in that battle but just before he dies, he sais to Bilbo "Farewell, good thief … I go now to the halls of waiting to sit beside my fathers, until the world is renewed. Since I leave now all gold and silver, and go where it is of little worth, I wish to part in friendship from you, and I would take back my words and deeds at the Gate." He also acknowledges a few lines later Bilbo's qualities - "some courage and some wisdom, blended in measure. If more of us valued food and cheer and song above hoarded gold, it would be a merrier world".
Late insight some would think , but insight it is and he gains it through his mistakes and his greed which he finally admits has led him to nowhere.
So what can we say about Thorin?
A hero with flaws and weaknesses, but one who is able to learn and manages to make up somewhat for the wrongs he has done, yes eventually redeems himself.
"Farewell, King under the Mountain".