Balin was part of the company assembled by Thorin Oakenshield to kill the dragonSmaug and retake the mountain kingdom of Erebor. He is the only one explicitly said to have been at the Mountain prior to the coming of the dragon. Curiously, The Hobbit states he was companions with Thorin on the day the dragon came, but according to Appendix A in The Return of the King he was only 7 years old while Thorin was 24.
He was the second dwarf to arrive at Bilbo's house at the beginning of The Hobbit, preceded by his brother Dwalin. Like Dwalin, he played a viol.
An artist's rendition of Balin's tomb. The inscription as devised by Tolkien translates to "Balin Son of Fundin, Lord of Moria".
Among the dwarves in The Hobbit, Balin was the second-eldest dwarf (after Thorin), and so he spoke for the party when they were captured by the Elvenking, as Thorin wasn't present.
He is described as "always their look-out man": He spots Bilbo approaching the Green Dragon Inn at Bywater, he spots the trolls' fire, and he is the first to spot the elves in Mirkwood. Naturally he could not notice Bilbo (hidden by wearing the One Ring) as look-out for the company after escaping the goblins in the Misty Mountains, and after this incident he came to respect Bilbo's abilities. He was the only dwarf who volunteered to accompany Bilbo down the secret passage to Smaug. Of all the dwarves in the quest, he was the only one Tolkien describes as visiting Bilbo afterwards at Bag End.