Throughout the story campaign, you will meet different survivors coming from different backgrounds. Once you have gained their trust and friendship, some of them can be unlocked as playable characters. Much better, some even have special Traits and Skills which will prove beneficial for your community.
During the campaign, the character you are currently using will eventually get tired or injured. Just leave the character at your Home Base and pick another character to head out. The resting character will recover in about 24 hours, just make sure your supplies are stocked. To swap survivors, choose the character in the Journal and check if there is a message indicating if that character can be Switched To.