One of Nat’s dark secrets was revealed and she went on the run, alone. Hunted by just about everybody, SHIELD included. Who turned up to offer his help?
Captain America: Prisoner of War
Complicated things happened and Bucky ended up in a gulag. After enduring it for far too long, he escaped during a riot. And who was there to make sure he didn’t end up dying in the snow?
Captain America and Bucky: The Life Story of BuckyBarnes (2012)
Even back when they were a part of the Red Room, their masters couldn’t stop Nat from going against orders and helping Bucky out.
Black Widow (2014-15)
After their relationship was erased from Nat’s memory and she was freelancing, who just happened to drop by?
(While he had a legit excuse for the first time, this wasn’t the only
time he popped up during that run. He explicitly kept an eye on her and
stepped in when she needed him to.)
She may not have asked for his help, but when has that ever stopped
either of them?
Black Widow (2016-17)
And when Nat was out in the cold (again), who watches over her (again)?