Balin is a dwarf leader and elder brother of Dwalin. He goes on Bilbo's quest. He also becomes the dwarf king of Moria.
Balin was a close advisor and friend of the royal family of the dwarves, especially the king's grandson and heir to the throne Thorin. When Smaug the Dragon attacked Elebor, the dwarves homeland, Balin's life was saved by Thorin. Along with the rest of the dwarves, Balin fled the Lonely Mountain and tried to find a new homeland. Balin fought in the Battle of Azanulbizar, when the dwarves tried to take the mountain lands of Moria for their home from the orcs who had overrun it centuries before. The battle was victorious and Balin settled in Moria.
When Thorin and the wizard Gandalf decide it was time to take the Elebor back from Smaug, Balin tried to convince the dwarves in Moria to join Thorin on his quest. The Moria dwarves refused. Balin brought this news to Thorin, and Thorin requested Balin join him and eleven other dwarves (including Balin's brother Dwalin) in taking back the Lonely Mountain alone. Balin questioned the wisdom of this, but he said that Thorin was always his king and he would follow him if Thorin wanted it.
On the quest, Balin is the most loyal to Thorin, and constantly defends him when Thorin's mood swings anger the other dwarves.
After the events in THE HOBBIT, Elebor was taken back, after Thorin and his company fought both Smaug and a horde of orcs led by Azog the Defiler. Thorin died in the final battle. Unlike most of the remaining dwarves in Thorin's company, who decided to restart their civilization in Elebor, Balin returned to Moria.
After a number of years, Balin became king of Moria. He ruled successfully for at least forty years, but when the ancient evil Sauron returned to power Moria was overrun by orcs, who were accompanied by a powerful demon known as the Balrog. Balin and his people fought valiantly for many days, but in the end they were all killed in a massive genocide.