Nathi was born in Erebor long ago, just before the dragon, Smaug raided and claimed it.  When the worm came, Nathi and his family got away safely, for they were out on the mountain-side prospecting.  They saw the dragon from afar and warned those they could find of the crisis at their doorstep. 

And so, he and his family fled to the Blue Mountains, where Nathi learned his father's trade, armoursmithing.  He's become quite skilled at this craft.

Now, seeing both his parents die, and reaching the very respectable age of 230, Nathi wishes to return to Erebor, hopefully to die there peacefully, in his true home.