
Below are some places where you can spend your crystals.
Starter Hero Packs
The game will offer you 'hero' packs when you begin that contain multiples of Heroes Biometric. If you are a free player spend your crystals on these until you have established a decent hero roster.
Uniforms
Depending on the hero and the uniform you can get special buffs and stat increases when you spend crystals on uniforms.
Energy
If you are a paying player and have reached a high Shield level in the game you can spend your crystals on getting extra energy for your roster.
Continues and Gold
Avoid buying 'Continues' and gold as they are both a waste of crystals.