These characters come in presets, and if any die, you can choose from the others in the list, or swap them out any time you are back at Ranger HQ.
The good news is that even while you’re off adventuring, they’ll be increasing in level as well, so you won’t have to worry about re-training them.
After choosing from the initial 2 main characters (or customizing your own), you will arrive at the Ranger HQ in Colorado, and once you’ve situated yourself in command, you can speak to your new friends to manage a list of potential Ranger recruits.