In the lead-up to Avengers: Endgame, Black Widow was the biggest female superhero in the Marvel Cinematic Universe. After Scarlett Johansson's Avenger debuted in Iron Man 2, Natasha Romanov proved an essential companion to Captain America and the rest of the heroes, atoning for her largely unseen sins over the course of several films. While Natasha did prove a vital part of Avengers: Endgame -- after all, without her, the Soul Stone would never have been retrieved -- she didn't have as big a presence in the film as many going in might've expected.
While Natasha's story came to a close in Endgame, a surprising heroine proved to be the real MVP of the film. In many ways, she ended up having the biggest growth over the course of the film, with arguably the strongest emotional resonance throughout. Instead, the cosmic former villain Nebula had the greatest impact on the whole story and stands as the film's true female lead.
Nebula Before Endgame
Nebula, up until this point, proved a somewhat nebulous character. She started off as a glorified henchman to another glorified henchman in Guardians of the Galaxy, before taking on a somewhat more prominent role in Guardians of the Galaxy Vol. 2. Despite playing a supporting role in each, the core tenants of her character shone through: she has a complicated relationship with her sister and had a one-sided, abusive relationship with Thanos. While she still desired his validation, she was ultimately willing to turn against her adoptive father, Thanos.
Her arc in Guardians of the Galaxy Vol. 2 brought her to the point where she'd make a futile attempt to face Thanos on her own, as seen in Avengers: Infinity War. Still, as one of the few cosmic heroes not snapped out of existence, she ended up being one of the few people left alive following the Snap. Even more so, she was stuck on Titan with Tony Stark, a person she has never interacted with until that point.
While Nebula had nearly no screen-time in Avengers: Infinity War, many of the characters who had a huge presence in that film had little presence in Endgame. so Nebula was well-positioned for a meaningful arc in Endgame.
The Start of the Path
Shortly after the cold open on Hawkeye's ranch, Endgame begins by focusing on two core characters of the film to come: Tony Stark and Nebula, as they work in tandem together in order to forge a path home again. In prior films, Nebula had only received kindness from Gamora, her now-dead sister. But Stark shows her empathy and warmth, even showing self-sacrifice in the face of inevitable death. She, in her various small ways, reciprocates this kindness, and this kindness is key to her arc throughout the film.
Even her arrival on Earth, though under horrific circumstances, leads her to more kindness. She lost almost everyone she knew in the Snap, more than any other single character. She ends up finding a new one of sorts in the Avengers, finding acceptance from Rocket Racoon (the only remaining member of the Guardians), War Machine (who is similarly augmented by metallic augmentations) and the rest of the Avengers.
While all of the Avengers find companionship with one another in the half-depopulated world, Nebula finds the most companionship, which makes it even crueler when she's forced to confront her past.
Black Widow's Clean Ledger
By comparison, Black Widow is barely a presence in the film. Black Widow's arc is mostly complete by the time Endgame starts. She's a former assassin who had committed so many sins in her lifetime that she feels beyond atonement. For her, Captain America: Winter Soldier proved her greatest instance of development, as it allowed her to see that she has worth outside being an assassin, that people value her and her needs.
By the time we start Endgame, she's managing an inter-planetary task-force that manages the world. In essence, she's become the second-in-command of the Avengers, becoming a pseudo-Nick Fury in the process. She lives for her family, and she feels home here, despite the stresses that come with the job.
But while this is a stage in her general arc, she doesn't truly grow from here. She starts at this point, remains at this point, and all actions she takes from here stem from the point she's at where she starts the story. This dedication to others especially comes through when she sacrifices herself to gain the Soul Stone. In doing so, she helps Hawkeye move on with his arc. She completes her arc, which is encapsulated by her funeral and memorial service before Hulk uses the Gauntlet.
Black Widow's entire arc in the film establishes the Avengers are her family and that she is sacrificing herself for someone else as a final service to those who gave her life meaning. Despite playing such a huge role in the Avengers, she plays a surprisingly small role in the narrative, and her presence in the film, until she sacrifices herself for the Soul Stone, is small.
Compared to Nebula, Black Widow feels minor in Avengers: Endgame's plot.
Nebula's Revelation
However, the entire third act of Endgame could not take place without Nebula. If Nebula didn't cross paths with her past incarnation, past-Thanos and past-Nebula would never have reached the current era. This means that the sins of Nebula's past effectively result in Iron Man being forced to sacrifice himself.
This time travel aspect also reveals what could've been, if Thanos had gained power over the Infinity Stones earlier than he did. The past Nebula, feeling trapped under Thanos's glove and desperate to please him -- if even for her own sake -- ends up bringing Thanos to the future to retrieve the Gauntlet.
It is only when Nebula gains the trust of her sister Gamora and kills her past self that we see just how far Nebula has come as a character since her first appearance. She would have destroyed the world for him, but by the time Infinity War started, she's willing to die to stop him.