Hint: Ranking System
To get an S ranking beat the game in under one hour and thirty minutes. You cannot use special weapons, weapon upgrades, and only one herb. Also, only pick each weapon up one time.
To get an A ranking, beat the game in under three hours and don't use any First Aid sprays or special weapons. (You can use herbs, though.)
To get a B ranking, just beat the game in under three hours.
To get a C ranking, finish the game in five to seven hours.
For a D rating, finish the game in seven hours or more. Note that your rank will be lowered by one grade if you use any special weapons or if you saved more than 12 times.

Hint: Sub-Machine Gun
Play the first scenario with either character. Enter your finger print on the computer in the giant moth room. When you get to the door where you need your finger print to enter, enter your finger print and choose not to continue. Finish the game and save. Play off of the game you saved with the next character and follow the same process and put in the second finger print. The door will unlock and inside will be a few Lickers and a sub-machine gun.