1. Natasha/Bruce: To Natasha, Bruce represents a complete loss of control, and Natasha is all about control. She’s been trained since childhood how to use everything about the way she presents herself to the advantage of the mission at hand. The Hulk is terrifying to her not only because he could destroy her and her skills won’t work on him, but also because of that loss of control. She is a very good actress and controls her fear when she’s around Bruce, treating him with just as much politeness as she does any other Avenger, but when the Hulk comes out she loses it. She doesn’t let her fear stop her from getting the mission done though, going up to get Clint right after she’s running for her life with a damaged leg. Bruce pities Natasha just a little bit, but pity isn’t really the right term. Everything about Natasha is the opposite of pity. If you said you pitied her you probably wouldn’t live to take back your words.
2. Natasha/Clint: They’re friends and partners. He is probably one of the few people at SHIELD who knows about her past because she told him instead of finding out from a file – which, how much of that is reliable anyway? Natasha’s the one who lived it; she knows the truth of her own life. She trusts only a few people with her life, and Clint is one of them. She’ll do what it takes to keep him safe – the meaning of the word changes in their line of work, but the feeling behind it remains constant – and he will do the same for her. She’s pretty sure he had a crush on her when they first met; he flirted with her constantly even after she repeatedly kicked his ass in a variety of ways. That phase is over now, he realized that she was not interested in him in that way and settled for being her friend. If Natasha was the type to have a best friend, Clint would be it. They’re both performers, though historically for very different audiences, and they have nothing to live for but their work, which is ultimately all they care about. Their work and each other.
3. Natasha/Steve: Natasha is certain that it’s impossible to have friends and romantic partners while living the life of a SHIELD superhero (it’s going to be a while before she can think of herself as a superhero, but she’s already being lumped into the Avengers and it doesn’t seem to be going away). Steve is probably the most heartbreaking example of that theory – he had it all for a short while, and then lost everyone he cared about to the past. Natasha thinks maybe that’s a punishment. She’s got a better technique, which is not to let herself get very close to anyone. Except Clint, but he understands. Steve is her caution sign; he’s exactly what happens if she were to let herself forget who she is – what they made her.
4. Natasha/Thor: She doesn’t really know what to think of Thor. He’s a god of thunder. From another planet. In her line of work, Natasha’s learned to take such things in stride, but accepting that he exists and knowing how to speak to him or relate to him are two very different things. Sure, he’s an outsider, but so is everyone else in this group of extremely unusual characters. He never really gets to see her in action when they’re battling Loki in New York, and when they finally do get to fight together, he doesn’t underestimate her as most men seem to. She’s so surprised that she actually asks him about it later, and she’s treated to a half-hour long speech in which Thor exalts the glorious victories of a lady warrior called Sif.
5. Natasha/Tony: Tony fits a cliché pretty much exactly. Natasha knows everything she needs to know about him before she’s ever met him. It’s interesting to see him work as a member of a team instead of a leader, a lone hero out to do what he feels like doing. She’s intrigued by the way Pepper grounds him and the way that he is clearly annoyed by Coulson when he’s alive but obviously upset when he’s killed. Natasha watches, and she files away all of the insights she observes for future use. It will come in handy, she needs something to use against him next time he hacks into the plane she’s flying and bombards her with loud, annoying classic rock.
6. Natasha/Loki: Sometimes if she stops to think she’ll realize the hopelessness of the situation, so she won’t stop to think, she’ll just act. She’ll treat this situation just like any other and let her training kick in. She knows it’s magic and gods and aliens and nothing she was ever trained for, but her training is all she has, so that’s exactly what she does. Through some strange miracle, it works. She manipulates Loki into revealing an important part of his plan, and undoes Clint’s brainwashing by hitting him really hard in the head. It’s all she can think of to do, but it works. During the fight she shuts everything out and deals with what is right in front of her face. It isn’t until afterwards that the enormity of the situation dawns on her. She spends hours in the training room sparring with Clint, both of them fighting until they’re too exhausted to think anymore.