Natasha
Bucky carries Steve into the Tower, not because Steve cannot walk but because Bucky is not ready to let him go.
He settles his Omega into bed and for the first time leaves the room, he has to make sure they are safe, that no one will harm them here. Their child's assurances mean little but for now it is enough to keep Bucky from testing the Tower in it's entirety.
He pauses when he notices her presence and Bucky wonders if Sam sent her.
He watches her move in the darkness before she steps out of it.
Bucky's breath catches because this one is his, this one he chose first and it makes sense that she'd find her way to Steve, seek him out for the comfort and safety he could not provide.
Her instincts would have told her to trust him even before she knew him because he smelled like Bucky, smelled like home and safety, what little Bucky had been able to afford her when he'd trained those little girls.
Too many had not survived the training, like all the children that had not survived to birth, even fewer had survived their first mission and only this one had survived the escape to freedom.
"Natalia." Bucky said, he was not surprised that he had not recognized her at the beginning, too focused on the return of his Omega, on his protection.
The woman paused as she stared at him.
There were no more words exchanged but they knew, Natalia knew that he intended to keep Steve safe, Bucky knew that she intended the same and they both nodded in acceptance and agreement.
Bucky watched her turn to leave, walking after her to close the door behind her, he wondered if she knew that he'd adopted her as his child all those years ago, wondered if she knew that Steve had accepted that adoption the moment he'd seen her, for what reason Steve would not have known, he would have only known his instincts said this person was his to keep safe, to protect, that they were his child.
"Natasha." The Alpha said softly as she stepped out through the door, "My name is Natasha now." She looked at him, somehow earnest and pleading.
Bucky nodded, "Natasha. It is a pretty name for a pretty spider."
She stared at him and seemed to understand what he meant, nodding before leaving.
Bucky finished his rounds, securing the entrance Natasha had used but leaving the other one that bore her scent for her to utilize in the future should she require Steve's comfort or the security of Bucky's protection, he did however mark it himself so she understood that he was aware of it's location.
Then Bucky went back to Steve, drawing his Mate into his arms and whispering a thanks to him for protecting his little Ballerina when he could not.
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A/n: i just like the idea of instincts not always being something that can be explained. And Steve protects Natasha in both the Avengers and Captain America Winter Soldier more than the others and this is an explanation for that other than she's female.
So it his instincts saying that his Mate has adopted this kid so we have to protect her now, which means she's technically the first born.
It also explains why he allows the teasing but doesn't really trust her yet, parents don't have to trust their children.
Just so you know when Bucky refers to their child in the second paragraph he's talking about Tony because he doesn't yet know about the others.