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Okay, so Brock Rumlow had a building/helicarrier dropped on him. Let’s be honest, the only way he survived that is if Hydra had enhanced him somehow. Ozhawk had the idea that it might be animal like. I’m running with that because it’s awesome. Rumlow is enhanced to be similar to feral mutants (Wolverine, Sabretooth, Beast), he heals fast, but he’s got animal traits/reactions.
Maybe he’s got some sort of pack animal thing going on, like wolves or lions. And this is a problem for him because while he doesn’t really want to be Hydra, all of his fellow enhanced ‘packmates’ are Hydra and it’s like, even if all the constraints that are keeping him in Hydra (they’ll kill him if he tries to leave, where/who would he go to anyway?) go away, he’s stuck because his whole pack is Hydra.
Then we have Darcy. Darcy whose whole world has quietly imploded several times recently (first when her brother died, then when Hydra approached her, then when Cap blew Shieldra up). All she has right now is a desire to keep Hydra’s attention on her instead of Jane, and a new friend in Rumlow and an old friend in Sitwell. Darcy looks at Rumlow’s conundrum and says 'well that’s easy enough to fix, I’ll be your pack.’
And every time he or Sitwell tries to say that it’s not that simple, she makes it simple. She just sticks near Rumlow, and she provides the physical contact/reassurance his feral side needs, and she gives him her trust, and since she’s not a Hydra scumbag, it doesn’t take long before he trusts her too. And the feral part of him decides that 'yes, this small, fierce, angry person is my pack’ and basically imprints on her.
So now, when he’s struggling with control, her voice or pulse or scent grounds him and helps him keep control. Of course, it means that they pretty much have to stay close together, but Darcy is used to close quarters with questionably stable individuals and Rumlow has gotten used to needing to be close to pack. This gets them all set up for when they end up on the run and ridiculously co-dependent.