Unlockable Costumes

You can unlock more costumes by doing the following. Credits to the wonderful people in this Gamefaqs thread that compiled and shared their findings to help us fellow players.

Valentines Outfit (Both) - Complete story mode once

Duck Costume (Both) - Complete Boss Rush Mode within the time limit

Bunny Costume (Remi) - Clear Wildcard Spell mode

Bunny Costume (Choux) - Clear Wildcard Weapon mode

Police Uniform (Remi) - Destroy a training dummy.

Police Uniform (Choux) - Complete story mode once as Choux

Military Uniform (Remi): Completely story mode in Custom Loadout mode

Military Uniform (Choux) - Clear Hard Mode

Cheerleader (Remi) - Clear Nightmare Mode

Gym Uniform (Remi) - Complete Powerhouse Mode

Gym Uniform (Choux) - Complete Sugar Rush Mode