How do you get Ashton?

Ashton joins as part of the story if Claude is the one you chose to start with, but if you start with rena you get a better character... Diaz.

Hes awesome, his air slash ensures no one gets close enough to hit anyone. And it's only 4mp!

You get Ashton after you get to the port town of Hilton; it does not matter if you choose Claude or Rena. Though it is just my opinion, the best fighters in the game are Claude > Ashton > Dias, but you can get both Ashton and Dias if you want. Also, I think if you did the private action where you run into Ernest in Cross, you can no longer get Ashton. If you are at the point where you can choose for Opera to join, then it is too late to get Ashton.

Talk to man at the entrance of Herlie about a dragon at Salva, then return to Salva Drift and find/recruit Ashton. You do not need to reach Hilton, and I suggest you avoid Lacour continent until you have Ashton. If go there first, you might accidentally progress the storyline too much and lose your chance to recruit Ashton.