David Harbour's character in the Black Widow movie is the Russian Captain America. The Marvel Studios panel at SDCC 2019 officially confirmed Harbour's long-rumored involvement in the movie, and not only that but revealed just who the Stranger Things star will be playing in the Black Widow movie: Red Guardian.
Although Red Guardian may not be the biggest name from Marvel Comics, he is an important one and a particularly interesting choice for the MCU's Black Widow movie, because he's the Soviet equivalent of Captain America. The character, real name Alexei Shostakov, was first introduced in Marvel's The Avengers #43 in 1967, with the Soviet Union secretly creating its own version of Captain America because of the brewing Cold War with the United States. He's a master of hand-to-hand combat, an expert pilot, and highly trained in espionage.
As you might expect, Red Guardian has previous with Black Widow and Captain America, having gone up against them both, but it'll be interesting to see just how that translates to the present-day MCU. Although there's definitely still something to be explored with regards to the relationship between Russia and America, this isn't the Cold War, and Natasha came up as a spy long after that, so the character will have to be updated to fit in with the more modern setting. However, it can still retain his own loyalty to Russia contrasted with Natasha Romanoff working for the U.S.
Since Black Widow is taking place between the events of Captain America: Civil War and Avengers: Infinity War, it means we're not quite getting the Red Room prequel fans had perhaps envisaged. The movie will still be exploring Black Widow's past and how it comes catching up to her though, which will happen with Florence Pugh's Yelena Belova, and can be seen with Harbour's Red Guardian too. After the events of Captain America: Civil War, Natasha is working as a rogue agent having sided with Team Cap and gone against the government. It's likely that lots of spy organizations are going to be looking for her, either because of what she knows or in the hope she might work for them instead, either of which would fit with Russia.
During the Marvel Studios panel at SDCC, Harbour wore a Captain America t-shirt, which was very much a knowing choice, and mentioned having an interesting story with that character. Given that the Red Guardian was designed to be Russia's Captain America, and he'll have watched as Cap become a beloved superhero and then fell to become essentially a criminal in the eyes of the government, which might make him question his own place in things too. Since Harbour is among the principal cast but isn't playing Black Widow's main villain Taskmaster, we can expect he and Black Widow to have quite a complex relationship.
The time between Captain America: Civil War and Avengers: Infinity War is defined by her choice in the former, upon which Captain America was a major influence, just like he was in the decision to return to the fight in the latter. By having someone who closely represents another version of Captain America, but mixed with her old life in Russia, there's a great way for Harbour's character in Black Widow to reflect the past of Natasha's story while driving it forward towards Avengers: Infinity War and, eventually, her sacrifice in Avengers: Endgame.
