Crossbones

Brock Rumlow was a HYDRA sleeper agent inside S.H.I.E.L.D., and a STRIKE team commander. He is a world-class hand-to-hand combatant, with extensive experience in various street fighting, martial arts, and military combat techniques. During the HYDRA Uprising, Rumlow led the HYDRA team attempting to assassinate Captain America, until Rumlow was nearly killed during the Battle at the Triskelion.

Once he recovered, and hearing that Alexander Pierce had died, Rumlow escaped from the hospital and, assuming the codename Crossbones set on a path to extract revenge and make anyone involved in his downfall suffer the same pain and loss he endured.