Unlock Characters:

Beat the following levels on Story Mode under Medium difficulty:

To Get        Beat

Ozor Mox      2280 Return to Planet X
Sadako        2019 NeoTokyo
The Colone    l1853 Wild West
Khallos       1972 Atom Smasher
Stone Golem   1920 Aztec Ruins

Get Streets Arcade level and Cardboard Characters:

Beat the game under Easy difficulty

Get Lean Molly & Hector Babasco:

Get a gold medal in Adios Amigos on Arcade mode.

Unlock Sewer Zombie:

Get gold on Behead the Dead level.

Thanks to Revolution reader Bearfood06 and Maxxi!