Natasha Romanoff a.k.a.Black Widow (Scarlett Johansson) made her debut in the Marvel Cinematic Universe assisting Tony Stark (Robert Downey Jr.) in Iron Man 2, and after joining The Avengers, joined forces with Steve Rogers (Chris Evans) to save the world in Captain America: The Winter Soldier. This summer she re-teams with Earth's Mightiest in Age of Ultron, and afterwards must pick a side in Captain America: Civil War.
The third Captain America picks up from the events of The Winter Soldier and Avengers: Age of Ultron, and as of today we can confirm that the entire core cast of Cap 2 will be back for Civil War - including Black Widow.
It comes as no surprise that Black Widow will be back for Civil War alongside Falcon (Anthony Mackie) and Bucky (Sebastian Stan), but it was confirmed by directors Anthony and Joe Russo today during a press event in China where they seem to be scouting potential locations and talent, given that China is the second most lucrative film market. That's why Iron Man 3 made a point of shooting their and including exclusive (and unnecessary) footage that only played in Chinese theaters.
As we've expected from the get-go and from chatting with the Russos (and having them on the podcast), Captain America 3 was always going to be an all-out Avengers movie despite its title and you can expect many more notable heroes to appear as well, including new characters introduced in Avengers 2 and potentially even some familiar faces from the Marvel TV side. Much like The Winter Soldier drastically altered the landscape of the MCU by eradicating S.H.I.E.L.D., Civil War will do the same and potentially worse.
What makes the Black Widow character particular interesting is in what role she can play in that larger story. Civil War is just that, a war that pits heroes vs. heroes when in Marvel Comics, Iron Man and the government attempt to force all heroes to register and reveal their secret identities. Captain America disagrees and a line is drawn between two factions as the rest of the Marvel universe is forced to pick sides. In the comics, Spider-Man is the key character stuck in the middle of the conflict.
In the movies though, the Civil War story is going to be adapted in a way that'll see the plot play out very differently than in the books since there aren't enough heroes in in the MCU yet to introduce a Superhero Registration Act and Spider-Man is currently owned by another studio in Sony. That's been a topic of much speculation for several months since the infamous Sony hack revealed leaked emails confirming that Marvel and Sony were at least in early talks about that would potentially let Marvel introduce a new Spider-Man in Captain America: Civil War, but as far as we know, it's too late for that to happen and if Spider-Man does come over, it'll be in The Avengers: Infinity War- Part 1.
Instead, Captain America: Civil War introduces a new important Marvel hero in Black Panther who will get his own film in 2017. During the Marvel's presentation where they unveiled Civil War and Black Panther, they teased that it would be this new character who's caught in the middle of the Captain America-Iron Man conflict, but we feel that Black Widow may have a similar role as well.
Romanoff is loyal and respectful of Captain America and what he stands for, especially after what they've been through. On the other hand though, if Captain America is loyal and forgiving of his old pal Bucky Barnes, a.k.a. The Winter Soldier, Romanoff has an inherent distrust of the assassin who previously put a bullet in her and almost took her out again in Captain America: The Winter Soldier. It was hinted in that movie that The Winter Soldier might have been involved with the death of Iron Man's father, Howard Stark, as well, which might be one of the sticking points of conflict in Captain America: Civil War.
It won't be long until we learn more since Captain America 3 begins shooting in April.
The Avengers: Age of Ultron releases in theaters on May 1 2015, followed by Ant-Man on July 17 2015, Captain America: Civil War on May 6 2016, Doctor Strange on November 4 2016, Guardians of the Galaxy 2 on May 5 2017, Thor: Ragnarok on July 28, 2017, Black Panther on November 3 2017, Avengers: Infinity War - Part 1 on May 4 2018, Captain Marvel on July 6 2018, Inhumans on November 2 2018 and Avengers: Infinity War - Part 2 on May 3 2019.
Source: SINA (via CBM)