To unlock the following Secret Skins, complete the following tasks.

Unlockable	How to unlock
Ballhead 2 Raptor	Clear Arcade Mode with every character.
Ballhead Raptor	Clear every Story mission with every character in Story Mode.
Dark Safety Weapon Doombox	Reach 1,000,000 u/s ball speed outside of training (must have Safety Weapon Doombox unlocked first)
Domino the Blessed Dice	Unlock all of Dice's other skins.
Domino the Cursed Dice	Cycle through Dice's skins in the character select screen very fast until you unlock it.
L.2 Full Mecha B Latch	Clear Arcade Mode with Latch without continuing (may require L.2 Full Mecha Latch to be unlocked first)
L.2 Full Mecha Latch	Clear Arcade Mode with Latch without continuing, then clear Green 1: The Offer in Story Mode.
Noise Clone Candyman	Reach level 31 in multiplayer.
Noise Clone Dice	Reach level 36 in multiplayer.
Noise Clone Doombox	Reach level 56 in multiplayer.
Noise Clone Grid	Reach level 51 in multiplayer.
Noise Clone Jet	Reach level 41 in multiplayer.
Noise Clone Latch	Reach level 26 in multiplayer.
Noise Clone Nitro	Reach level 46 in multiplayer.
Noise Clone Raptor	Reach level 11 in multiplayer.
Noise Clone Sonata	Reach level 21 in multiplayer.
Noise Clone Switch	Reach level 16 in multiplayer.
Old Workbot Frame Switch	Clear Arcade Mode with Switch without continuing.
Safety Weapon Doombox	Clear Story Mode.
Straitjacket Alert Candyman	Clear Orange 1: Tests in Story Mode without being scored upon (requires Straitjacket Candyman to be unlocked and purchased first)
Straitjacket Candyman	Clear Arcade Mode with Candyman without continuing.
Workbot Frame Switch	Clear Blue 4B: The Mayfly in Story Mode without losing a stock.
Zoot Suit Deluxe Sonata	Play a match after reaching maximum Spark capacity of 9,999.
Zoot Suit Sonata	Clear Orange 4: Getting Up in Story Mode with Sonata.