Okay, I was doing the Dustia trick (where you use Phoenix Downs on Dustia to gain a crapload of MP). Anyways, at one point I kind of stayed too long for the kill to see it, and then when I went back to the previous area and proceeded back to the Corriders of Sand, he was gone. My guess is that I stayed too long and he died and wouldn't return.
My question is simple. Can Dustia return? Or is he gone forever?