The following characters become available when you complete the corresponding tasks and can be used in Survival modes. They do though however have different stats that will not carry over if they are used in Story mode.
Unlock Boogie: Buy everything in survival mode.
Unlock Bubbles: Survive 15 consecutive waves.
Unlock Cesar: Kill an enemy with a sink.
Unlock Chop Chop: Perform 60 pistol counter kills.
Unlock Classic Shank: Kill a goon by throwing a bomber.
Unlock Defender: Perform 100 turret kills.
Unlock Falcone: Complete Campaign on hard.
Unlock Hobo: Defeat all waves of a survival map.
Unlock Horror: Reach the zombie wave in each survival map.