Natasha is a spy, not a soldier, crafted in the heart of Russia to have a very specific skill set. It’s hard to see that looking at her. She is, after all, the legendary Black Widow. Her job requires her to constantly be on the move, one step ahead of everyone else, planning and pretending. As a shadow-slash-spy-slash-assassin for SHIELD, she's required to be someone she's not on a regular basis. As a result, most of the people she comes across only get to see what she puts out, which is that mysterious aura of ‘who is this woman?’. She tends to compartmentalize her emotions, because of this. It's easy for her to box up how she feels about something and ignore it for the sake of her sanity or the mission. It's not always foolproof, though; if you had two tons of giant green rage monster barreling down on you, you'd probably wet your pants and scream. Natasha doesn't. Oh, she's frightened, and it very clearly shows. But she doesn't let that fear consume her. She stands back up and gets the job done because that’s what needs to be done. Loki even tries to upset her, using past deeds revealed to him by Clint. It works, sort of. Natasha allows him to see her get upset to the point of tears, but it’s only a ruse (or so Loki thinks). She shuts off that pain and guilt, smiling sweetly at Loki and thanking him for his cooperation. Mining her vulnerability is a weapon Natasha uses as much as the Widow Stings on her wrists.
The past happened. Now she tries to make up for it. She earned the name Black Widow over the years, lending her skill set to the highest bidder. Lies, murder and betrayal were just part of the job, and for a while, Natasha herself though she was unsavable. These are past sins that she wishes to atone for. She keeps a ledger, a system of checks and balances, and it’s bleeding red, and Natasha would probably be the first to say that her hands are covered in blood as well. Her sins of the past are constantly weighing on her mind, and Natasha is the kind of person who, while she does not regret the choices she has made, because they have brought her to where she is today, she knows that in making those choices she has wronged people. Maybe she can’t set it right with them, exactly. But she can do other things to regain that sort of karmic balance. (Although she herself would not use the word karma, likely. If brought up at all, Natasha uses the ledger analogy, for the universal symbolism of it.)
Her life consists of checks and balances, now. It’s not something she’s willing to let go of lightly. Natasha's worked hard to cultivate a sense of self that did good, and it was ripped away from her. She's spent a lot of her life hiding who she is. Even people who seem to think they know the Black Widow don't really know her, passing her off as just another pretty face when that's what she's wanted all along. Her reputation precedes her, but that’s all it is. ‘The infamous Black Widow’, she’s called by a Russian black market arms dealer. He expresses disappointment when she’s not up to snuff. He’s proven wrong (rather violently) later. She likes it this way. It means no one knows what to expect from her, and she’s able to use the greatest weapon in her arsenal: herself. The mission always, always, comes first. Natasha will absolutely break cover when the shit hits the fan. It’s difficult to say if she loves her job. She certainly doesn’t dislike it, considering the amount of effort she puts into it. Natasha is one of the Agents Fury can trust to get the job done no matter what, and Natasha would probably agree with this. Loyalty, as a concept for Natasha, is an interesting thing. She’s a spy, and so she has to live in a world where she must be duplicitous at all times. This results in people not being certain of her, when she herself is certain of them. This works out fine when on an undercover mission, but when it comes to her personal life, it’s a little trickier to navigate. Natasha has very few people she considers friends, and even fewer she trusts with her life. She trusts Clint Barton because he gave her a second chance. She trusts Steve Rogers because he has twice now proven his worthiness to her, once when punching aliens in the face and again when taking down HYDRA. In essence, the quickest way to earn Natasha’s trust and loyalty is to trust her yourself. She doesn’t have that many people who do, and she knows this. She also knows that maybe she’s not worthy of it, and so she clings to those who trust her, which is why it hurts so much when she finds out they don’t. With that said, Natasha doesn’t expect your trust and loyalty. She knows how the spy game is played. She doesn’t have some naive ideal that everyone will be friends and holds hands. She knows backstabbing, literal and figurative, is a part of it. It’s as ingrained and as natural to her as breathing. When she's not on a mission, Natasha is sensible, down to earth and unafraid of banter (although she’s not exactly afraid of banter on a mission, either). She's a voice of reason among her teammates in SHIELD, and more often than not, she's the one keeping a cool head under pressure. She's quite observant, too, able to deduce even minute personality quirks after spending a short amount of time with someone, and file those away for further study. Her loyalty to her team, and to the man she feels helped her when no one else would, comes before all. She has her own way of doing things, a moral compass that might seem a little bit skewed to observers. She lives among shades of grey, and that’s how she views the world.
She has a sarcastic witty side, and she can be quite stubborn when pressed to do something that she doesn't want to do. Natasha’s not afraid to express her displeasure, whether vocally or through expression, when she hears or sees something she doesn’t like. She listens to orders from her superiors, of course, and rarely questions them. Her relationship with her colleagues means that she can, however, speak up when something’s bothering her with little fear of repercussion. Natasha is never afraid to do the right thing, even if the right thing isn’t very nice. In Fury’s own words, “Romanoff is okay with everything.” Obviously, this is proven not quite true, but out of everyone Natasha’s worked with, she’s got the most morally dubious compass. She has no problem doing the things that other people won’t do, because she knows they need to be done. After all, if anyone is going to do it, it might as well be her. She’s been getting her hands dirty since she was a child, and while she’s on a mission to atone, she’s sure of one thing: if she does the dirty jobs that no one else will do in the name of good then she’s working towards that atonement.
The spy world may not be the most comfortable of jobs, and it’s kind of lonely, if you really sit down and think about it. Becoming an assassin-slash-spy for hire probably wasn’t the best life choice, but it was hers. Choice and control are huge for Natasha, and she is adamant about keeping it that way. The unknown is her greatest fear. If she can’t analyze it, use her weapons (physical and mental) against it or kill it, then she doesn’t know what to do. Her skill set is what she has and what she knows, and she’s alright with that. Because she’s chose that. Yes, she may have chosen it during a tumultuous period in her life. But even now she doesn’t regret the fact she made that decision, even if she regrets the things she did after she made it.