Crossbones is Brock Rumlow - a henchman of the Red Skull and enemy of Captain America. He is an expert hand-to-hand combatant, an expert marksman and a skilled driver and pilot. He wears a mask due to the fact that his face is heavily scarred, although the circumstances of how that scarring actually originally occurred has not been revealed. That said, there was an occasion when he was exposed to the Terrigen Mists (the source of the powers of the Inhumans) and it gave him the ability to generate a circle of energy in front of his face which could fire energy beams capable of piercing and burning his targets. Upon first usage of this power, his mask burnt away to reveal his heavily scarred face, so this may have contributed to the full extent of his current scarring.
