WARNING: The following contains spoilers for The Falcon and the Winter Soldier Episode 5, "Truth," now streaming on Disney+.
On The Falcon and the Winter Soldier, John Walker was just recruited by a new character who appears to be a future major player in the Marvel Cinematic Universe, Countess Valentina Allegra de Fontaine, played by Julia Louis-Dreyfus. Much like Nick Fury showed up in Iron Man and mentioned that he was putting together a team, Val told John Walker he should be ready to pick up the phone when she called.
While it remains to be seen whether she'll morph into an ersatz Fury in the MCU, Val and Fury have a long-standing history in comics, and it's one that she manipulated Captain America into the middle of.
In the comic book Marvel Universe. Fury trained Val, and the two developed a romantic relationship as well. Val showed up for the first time in Strange Tales #159 by Stan Lee, Jim Steranko and Jerry Feldmann. That issue saw Nick start a relationship with a woman named Laura Brown, who he made into a S.H.I.E.L.D. agent in that issue. In that same issue, Nick Fury and Captain America sparred in front of other S.H.I.E.L.D. agents that Fury was working with, and Val introduced herself by knocking Fury on his butt.
Over time, Fury and Laura broke off their relationship, and he ended up dating Countess Valentina Allegra de Fontaine instead. The two even moved in together, but there was one problem. Fury never seemed to get Laura Brown out of his mind. He kept going back to Laura, and finally, Val was tired of it. Leading the team known as the Femme Force for S.H.I.E.L.D., alongside Sharon Carter, Val decided it was time to make Nick Fury jealous. She flirted with Captain America, making her an enemy of Sharon Carter and causing a break in Fury and Cap's relationship.
This started in Captain America #145 by Gary Friedrich, Gil Kane, John Romita and Artie Simek. Captain America was fighting against Hydra, but he was getting sidetracked by the drama on the side. Nick Fury was frustrated when the Femme Force showed up without Sharon Carter, but it turned out it was Val causing the problems. With Val and Nick a couple, it made things tough for Captain America's girlfriend. This led to Val stepping over the line and making flirtatious comments toward Cap, calling him a "hunk of man." She even kissed Captain America on the cheek in front of Sharon.
However, her flirting with Captain America was all it took to drive a wedge between him and Nick Fury. Nick and Steve first fell out in Captain America #149 by Gerry Conway, Sal Buscema, Jin Mooney and Shelly Leferman when Cap refused to join S.H.I.E.L.D. as an official agent, and Nick took it personally. He kicked Cap out and gave a widespread order that no S.H.I.E.L.D. agents could aid Captain America, including Sharon and Val. By Captain America #152, Nick Fury had heard about Val and Steve, and he was ready to stand up for himself.
This led to the next issue. The first time readers saw Countess Valentina Allegra de Fontaine was after Nick Fury and Captain America had a sparring fight, one that left Fury down and out. She rushed down and helped Fury after the fight. In Captain America #153, the two men fought again, and this time it wasn't a sparring bout. Nick Fury showed up at Captain America's place and fought for his honor. Once again, Nick was down and out at the end. Sharon and Val eventually showed up, and, once again, Val went to help Nick after the fight. She admitted she loved him and was only trying to make him jealous. It wasn't the last time Captain America and Nick Fury fell out, but it may have been the most dramatic.