New Weapons and New Clothes
Here is the way to get new weapons. In order to do this you have to beat a majority of the bosses with the character you want the new weapons for. Then when you beat the game and it shows the ending then the credits it will should say you have a new weapon. Then you'll have the option to save the game, save it and when the title screen reappears load the game you just saved and you'll start at the begining with the new weapon.
To get the new clothes beat the game and save it when it asks you to save it after the credits. Then when the title screen reappears load the game and all of your characters will start out with new clothes in their arms menu. Each character has 3 outfits including their original one. Some of the characters get new outfits when they equip their new weapons.