Use medium Difficulty level and beat the following levels to get the character indicated

2280 Return to Planet X - Ozor Mox
2019 NeoTokyo - Sadako
1853 Wild West - The Colonel
1972 Atom Smasher - Khallos
1920 Aztec Ruins - Stone Golem

Get Streets Arcade level, Cardboard Characters 

Beat the game using the EASY difficulty setting

Get Lean Molly & Hector Babasco

Use arcade mode and get a gold medal in Adios Amigos

Get Rotating Heads Cheat

Beat the pane in the Neck in Silver (Challenge Mode)

Unlock Sewer Zombie

Get gold on the Behead the Dead level

Get Mr. Giggles

Beat big top blow out with silver or better on elite league under frantic series

Get Crypt Zombie & Dr.Peabody

Beat the second stage of the first mission in the arcade league on gold to obtain Dr.Peabody and Crypt Zombie.

Unlock Feeder Zombie

In challenge mode beat Day of The Damned on silver to unlock Feeder Zombie