Leech Hunter:
To unlock the Leech Hunter mini-game, complete the game on any difficulty.

Leech Hunter Weapons:
Earning a rank in the Leech Hunter mini-game will give you bonuses to use in the game. Here are the rankings and what you will get:

E (1-29): Submachinegun Bullets
D (90-99): Magnum
C (60-89): Hunting Gun
B (30-59): Handgun
A (100): All Weapons

New Costumes:
Complete the game on Normal or Hard Mode, and you will find the Closet Key in Rebecca's inventory when you begin a new game. Take this to the room where you got the Hunting Gun and use it to get into a costume room.

Ending Bonuses:
Earning gameplay ranks will unlock ending bonuses, based on the amount of time it took for you to finish. The ranks and what they unlock are listed below:

D (9h01m or more): Leech Hunter and Closet Key
C (7h01m-9h00m): Leech Hunter and Closet Key
B (5h01m-7h00m): Leech Hunter and Closet Key
A (3h31m-5h00m): Submachine Gun, Leech Hunter, Closet Key
S (3h30m or less): Rocket Launcher, Leech Hunter, Submachinegun and Closet Key