The following glitch will not work online and you must have no Green Herbs in your inventory. To perform the glitch you need to find a Green Herb, pick it up and use it at the same time which is done by pressing A + Y on the default control. If this is timed correctly the game will try to subtract one Green Herb from a total of zero which causes the inventory counter to cyclically roll backwards to it's maximum amount which results in you getting 300 Green Herbs available for use.
A character's signature weapon becomes available for ALL other characters when you earn an 'SS' rank using that character on ALL missions up to mission 5-5. Alternately, you can spend 10 Nintendo 3DS Play Coins to unlock a character's signature weapons.