While it wasn't the most successful Marvel movie of 2014, Captain America: The Winter Soldier was the one that changed the MCU for good, with its final revealing act. Not only being just the setting for Avengers: Age of Ultron, the Russo Brothers' movie also hinted where the sequel will take us, by introducing Brock Rumlow.

For those that don't know, he later on becomes Crossbones, which is a very-very important character in Captain America's lore.

Who is Crossbones?

While he hasn't got any superpowers, Crossbones is a very skilled assassin, easily managing to kill lesser heroes and villains. He is very muscular, very athletic, and (on several occasions, in the comics) he has proved to be a worthy adversary for Captain America.

While in Captain America: The Winter Soldier we only saw him as Brock Rumlow, the ending (in which we see him barely breathing) hinted at his transformation in Crossbones.

Why is he so important?

In The Death of Captain America story arc, which is an arc following the Civil War one, Crossbones plays a very important part in, well, the death of Captain America (which, in turn, leads to Bucky Barnes taking on the shield and becoming the new Captain).

It isn't known yet whether the Russo Brothers will use this detail in Captain America: Civil War - however, there are strong chances of seeing this happen, since Chris Evans has said time and time again that he is ready to leave the MCU.

Is Crossbones really going to appear in Captain America: Civil War?

According to Frank Grillo (who has portrayed Brock Rumlow), yes - the actor said that he had to put on 15 pounds for Captain America: The Winter Soldier and that he will have to put another 15 for Captain America: Civil War.

In other words, according to Frank Grillo, his character will be even more vicious that in Winter Soldier.

Of course, it isn't known whether or not we will see the death of Captain America, so everything needs to be taken with just a pinch (and not more) of salt.