Dain is Thorin's cousin. When Thorin sends the ravens to inform him of his standoff with Bard and the Elvenking, Dain marches with five hundred of his men to back up Thorin. When the goblins and Wargs attack, Dain immediately goes to meet with the Elvenking and Bard to decide how to combine their forces. And after the Battle of Five Armies, when Thorin has been killed, Dain becomes the new King under the Mountain. Dain makes peace with the elves and Bard by handing over the share of the treasure Thorin promised in exchange for the Arkenstone. Dain then buries the Arkenstone with Thorin as a gesture of respect. He seems like an honorable guy, but we can't really say more than that – his role is important, but tiny.