Cheat Mode:

Rack up as many points as you can in the VMU mini-game to unlock the "Eternal Life", "Area Select", "Free Continue", and "Free Time Limit" cheat options in original mode. *Note: Exact number of points is unclear.

Blood Color Option:

Complete the game in arcade mode to open the "Blood Color" option.
Hidden Items in Biological Weapon (Industrial Sewage) Plant Level:
See the fan at the beginning of the Biological Weapon (Industrial Sewage) Plant level? Shoot it (you can not use a button) to collect some hidden items.

Different outfits:

Hold Start and press B, X, or Y when choosing your character to have a different outfit.

Level Select:

Select Fighting Mode from the main menu screen and hold start while you select a character. Now, after you choose the Fighting Mode, you will be asked what level you want to start on.

Free Play:

Complete the game in all three original modes to allow free play in arcade mode.

HINTS:

More Health:

In the beginning of the game if you turn left you will see a door. Go through the door and you will see a zombie with flies on it. Kill the zombie and shoot the middle stack of boxes until a box of health falls. This will give you lots of health.

Extra Stuff:

There are two doors in the sewage. One you can get double-pistols and in the other, there is a big box of health.