OK here is some info I got from Gamespot:

Redeemer will have a number of hidden and unlockable items, such as zombie cards, which can be collected after defeating lots of zombies. Collecting all three zombie cards unlocks the zombie player model, which will let you play as a zombie if you so choose.
-Gamespot

I'll keep a running total of the Unlockables.

Unlockables:
1. Playable Zombie

Here is some other news about unlockables that I can tell you.

There are two unlockable characters besides the original five characters that you can use at the beginning of the game.

sweet

you can unlock werewolf if you get one of the skills to level 99