The Survivors are all that is left of the world we once knew. They are a varied bunch, each holding on to a part of who they were before the zombies came, but they all know how to handle a weapon! Each Survivor has a different class too - the higher the class, the more robust the Survivor.
Survivors gain XP from doing missions, building and battling at night. Once they've gained enough, you can use coins to level them up. This increases specific stats of the character, which is dictated by the Survivors class and personality.
Leveling up survivors also increases their power towards the power rating which makes surviving nights easier.