Bonus characters
To unlock bonus characters you complete the game in teh following difficulties:

1. Easy difficulty -- to unlock a male character with a spread rocket as his special weapon.
   
2. Normal difficulty -- to unlock a female character with a hyper laser.
   

Weapons Upgrade
From the Title Screen, press X, B, Y, A, X, B, Y, A, X, B, Y, A, X, B, Y to enable a cheat that gives your weapons a faster recharge rate.

Cheats