Crossbones

"There are no prisoners with HYDRA, only order. And order only comes through pain. Are you ready for yours?"

- Brock Rumlow to Sam Wilson Crossbones is an enemy of Captain America in the comics. In the film Captain America: The Winter Soldier, he is an assassin who works for Alexander Pierce and HYDRA as a sleeper agent within S.H.I.E.L.D. and in this film he served as the hidden tertiary antagonist ,while in the sequel; Captain America: Civil War ,he appears as a minor villain.

Abilities
Crossbones is one of the world's best hand-to-hand combatants, with extensive training in street-fighting, military combatants, and various forms of martial arts, He has trained extensively with the Taskmaster to the point where he was actually made an instructor at the Taskmaster's school for criminals. Crossbones is also a superior marksman with guns, crossbows, and throwing knives. He also has some experience with torture and brainwashing, having effectively "reprogrammed" Sin and nearly so with Diamondback. He is also an expert pilot.

Avengers Assemble
He appeared in Avengers Assemble, he was captured by the Avengers and ordered to tell where the Red Skull is hiding. While Captain America wore his costume to infiltrate Cabal.

Marvel Disk Wars: The Avengers
Red Skull sends Crossbones to intercept the police van carrying the Celebrity Five in order to transfer their bio-codes to the Skull himself. Later, Crossbones meets with Mystique to get from her the DISKS containing Baron Zemo, Abomination, Graviton, MODOK and Tiger Shark, then betrays Mystique and gives the DISKs to the Skull.

At some unknown point, he is D-Secured and supplanted by Hawkeye.

Ultimate Spider-Man
In the episode "Agent Web", Spider-Man and Triton encounter Crossbones and his HYDRA Agents at the abandoned Inhumans city of Atarog. It is revealed that Crossbones captured Nick Fury to use as a bargaining chip in order to obtain Madame Web for HYDRA, but Spider-Man and Triton are able to save Fury and defeat Crossbones' forces after receiving assistance from Madame Web's precognitions.

In the episode "The Symbiote Saga" Pt. 1, Crossbones has become the acting leader of HYDRA at the time when Arnim Zola was missing. After Doctor Octopus was shackled up in Michael Morbius' lab after he double-crossed Arnim Zola, Crossbones orders Michael Morbius to experiment on the Venom symbiote sample that he obtained.

Captain America: The Winter Soldier
In this film, Rumlow actually served as the hidden tertiary antagonist.

Rumlow alongside Captain America and Black Widow were sent to retrieve the Lemurian Star, a ship of SHIELD, that was captured by mercenaries led by Georges Batroc. The mission was a success because they recovered the Lemurian and captured Batroc.

Later, Rumlow led the Strike team whose mission was to capture Rogers because Pierce declared Rogers an enemy of SHIELD. Rogers manages to escapes on numerous ocassions from Rumlow and his team but thanks to the intervention of the Winter Soldier, he finally manages to capture Steve Rogers, together with the Agent Romanoff and the former Armed Forces paratrooper, Sam Wilson. When Jack Rollins wanted to shoot Rogers, Rumlow stopped him saying "Not here" since the place was full of the news helicopters watching. He took the fugitives to a place where they could execute them but they escaped with the help of Maria Hill disguised as Strike member.

After Rogers said that HYDRA was still alive, Rumlow went the control center to verify the launch of Helicarriers when a launch technician refused to put the Helicarriers into orbit and said "Captain's orders", he was furious and threatened to kill him. Sharon Carter defended the Launch Technician by putting a gun to Rumlow's head. While a gunfight between HYDRA agents and S.H.I.E.L.D. agents is carried out, Rumlow managed to put the Helicarriers in orbit and escape.

When Rumlow was headed to the room where Pierce was, he is interrupted by Falcon. During the fight Rumlow acclaims that Wilson was no match for him. He manages to survive after a Helicarrier crashed into a building in which he was. Rumlow is last seen at the hospital for recovery as his body and face are smashed.

Captain America: Civil War
In this film, Rumlow has returned as the minor villain, under the alias of Crossbones, although this alias was never mentioned ,adopted or used by Rumlow and or any other characters.

After the events of the previous film, Rumlow seeked to draw out the Avengers and eventually to cause a devastating attack. Crossbones and some mercenaries participated in the theft of a biological agent at an Institute for Infectious Diseases (IFD) laboratory in Lagos, Nigeria. Crossbones' men drove a truck through the entrance before opening fire on the guards, filling the building with poisonous gas. Crossbones made his way through the building until he found a chemical weapon and stole it. However, they were soon confronted by the Avengers who interfered and fended off Crossbones' mercenaries before seeking him out.

As Crossbones left the building with the bilogical agent with him, he saw Black Widow attacking his men and this thing led him to charge her in a fight. Romanoff fought against the mercenaries before attempting to elecrotouce him with a Taser disk on his exposed neck. However, Crossbones informs her that he cannot feel pain anymore since his nerve system had been destroyed in the Triskelion crush incident, before locking her inside a tank and dropping a grenade inside the tank, eventually, this thing killed his own men, while the Widow barely managed to escape by using one of his mercenaries as a human shield.

Rumlow and his last remaining mercenaries got into their armored vehicle to escape from their attackers just as Falcon arrived and hunted them down. Crossbones gave the chemical weapon to one of his men before they crashed and departed, with each of the soldiers running in an opposite direction hoping to mislead the Avengers. Before long, all the men had been killed and the weapon had been secured and taken away by Redwing.

Rumlow then attacked and battled with Captain America, furiously attempting to beat him to the death for nearly causing to his own death in the previous film, while using his new battlesuit's upgrades to his own good. But, although that his new suit gave him such a massive strength that eventually rivalted to Rogers' ,he was unable to defeat or kill him when his opposite gained the upper hand in the fight when Captain America succeeded to rip off his mask helmet and his armored gauntlets ,led it to overthrew him for a chance in vicotry.

After the fight, Rogers demanded Rumlow to reveal for whom he worked ,and Rumlow claimed that it was Rogers' best friend; Winter Soldier, whose pulled his strings before arming a bomb applied in his vest ,intending to take Steve with him ,but then, Scarlet Witch used her powers to contain the blast as Rumlow screamed in anguish. She was able to whisk him away from Rogers but unintentionially placed him with a building with hundrends of Wakandan nationals. Rumlow had been killed ,and was among of those who perished in the explosion, right after Wanda could not mange to contain it.