Natasha Romanoff aka Black Widow was one of the more mysterious heroes in the MCU. By the time she sacrificed her life in Avengers: Endgame, there were still aspects about her that audiences didn't know. But despite her guarded nature, she did allow herself to get close to a few select people.
There were some characters with who Natasha had a history before she was introduced in the MCU. Some of her relationships formed over time as she began to trust her allies. Whether on the battlefield or sharing an intimate moment with each other, Natasha proved she could be a good friend.
Sam Wilson
While Sam Wilson is undoubtedly Steve Rogers's buddy, that friendship allowed him to get to know Natasha as well. They first met in Captain America: The Winter Soldier as both Steve and Natasha trusted Sam to help them when they were on the run.
In fact, much of the friendship between Sam and Natasha happened while they were outlaws as it is revealed in Black Widow that Natasha played a role in breaking Sam out of prison. Though both Sam and Natasha have people they are closer to, they banter and have fun with each other the way only true friends can.
Laura Barton
While the storyline involving Clint Barton's secret family was a small part of Avengers: Age of Ultron, it speaks to the friendship between Clint and Natasha. While the other Avengers are shocked by this new information, Natasha knows Clint's family very well.
Natasha is clearly a beloved family friend who is close to Clint's wife Laura as well. Natasha even expected Laura to name her next child after her until it turned out to be a boy. Laura is also able to pick up on Natasha's troubled mind, suggesting she knows her better than most
Okoye
Though Natasha and Okoye only met in Avengers: Infinity War, they seem to form a fast bond with each other. They especially have great chemistry on the battlefield as they team up together to help Wanda fight Proxima Midnight.
Following "The Blip", Okoye remains as a valued ally to Natasha, helping to keep an eye on the world that remains. She even seems to offer a calming voice of reason when Natasha begins to overreact to certain developments. It is a professional relationship but they seem to relate well to each other.
Bruce Banner
Natasha and Bruce first meet in The Avengers when she is sent to recruit him to the team. Though there is tension in that situation, it is also clear that they find some connection between them, both of them revealing a little about themselves to the other.
Though the romance between these two in Avengers: Age of Ultron doesn't really work, seeing them as a pair makes sense. Natasha even opens up to Bruce about her experiences in the Red Room and she is the only one who can help calm Hulk down.
Tony Stark
The relationship between Natasha and Tony Stark starts out a bit awkward. Tony is infatuated by her and more than a little creepy while she is spying on him for SHIELD. Despite getting off on the wrong foot, it doesn't take them long to find a mutual respect for one another.
Natasha and Tony almost seem to have a brother and sister bond. They get on each other's nerves and argue a lot, but it is always apparent that there is still love there. Given that they both sacrifice themselves in Avengers: Endgame speaks to how similar they were in the end.
Rick Mason
Though he has just been introduced in the MCU in Phase 4, Rick Mason establishes himself as a valued ally of Natasha. He puts himself at risk by helping her stay on the run and ultimately helps her in her mission to take down the Red Room.
Though Natasha is a bit standoffish with Mason, acting as if they aren't friends, she warms to him over time. He is the kind of loyal ally who is always there for his friends and never expects thanks in return. It would be interesting to see Mason return to the MCU and see his reaction to Natasha's death.
Nick Fury
Nick Fury is very much like Natasha in that he doesn't seem to be an easy person to get friendly with. He is secretive and deceptive as a necessity, operating alone and in the shadows. But among all the super people he associates with, it is clear he is closest to Natasha.
There is a bit of a paternal relationship between them as Fury looks out for Natasha while also being totally confident that she can look after herself. Natasha also cares a lot about Fury, showing genuine heartbreak and loss when she thinks he dies. They are two people who love each other even if they would never say it out loud.
Steve Rogers
Both Steve Rogers and Natasha have friends they share a deeper connection with, but they have been by each other's side more than anyone else in the MCU. They initially seem to make an odd pairing as Steve represents a simpler form of heroism while Natasha is more of a shadow operative.
However, they gradually begin to trust each other in situations where it is unclear who the real enemy is. Along with fighting together, they also comfort each other in times of loss and defeat, sharing a bond that goes beyond just being teammates.
Yelena Belova
It is safe to say the relationship between Natasha and Yelena goes deeper than just a friendship. In their youth, they saw each other as sisters until that reality was torn apart. As a result, their reunion is a little tense only for their love for each other to shine through once again.
Natasha and Yelena maintain their sibling rivalry, but there is a friendship aspect there as they can tease each other and still fight side-by-side. Though Natasha tries to run from her past, she eventually admits to herself and Yelena that they are sisters, even if it is not by blood.
Clint Barton
Natasha and Clint not only share a long history together, but they also form one of the best friendships in the entire MCU. The fact that they are the most ordinary of the original Avengers team seems to cement that friendship as they go on these massive world-saving adventures.
They have helped each other find redemption, with Clint recruiting Natasha to SHIELD and Natasha bringing Clint back from his dark Ronin path. Their final scene together as they each try to sacrifice themselves over the other speaks to the immense love they share.