Tony
Tony tolerated Pepper leaving him only because he knew she was his and she would forever be his the way she would never be Agent's. He had created her and he knew her secret.
When she came to work with him, she was timid and insecure. But she was bright. She had always been bright. And she understood him better than anybody had ever understood before, better than he had thought possible, right from the beginning. So he kept and trained her and made her what she was today: the world's richest and most powerful woman. Well, the most powerful person. She had not only money and power which came with it, but also insight the world so desperately needed. She was the futurist. She was the one who saw where the world was going (Tony had stopped caring a long time ago if he ever had) and she drew the course.
She had left him, not for the other man, but for Stark Industries. To focus on her job. To rule the world with all the resources he had given her. Agent's only purpose was to help her to relax so she did her job well. And Tony loved her for doing her job well. Among other things.
She had saved his life. Several times. And above all, she made his life easy. Well, at least now, when they weren't together anymore. When they were, they just stressed out each other.
He knew the difference now when he was with Darcy. A woman who was almost opposite to Pepper. Almost.
Tony compared all the time.
Pepper was kind. Darcy was, at least sometimes, mean.
Pepper was reserved. Darcy was openly sexual.
Darcy played with fire. Pepper was the fire most people didn't know how dangerous it was to play with.
Both of them were very intelligent, both from the middle class.
But they had striking differences, too. Pepper was on the autism spectrum, which she found out quite late in life. Darcy had been diagnosed with ADHD as a child. Tony had certain traits of both disorders.
Pepper was easy to teach. She took well her new status next to him, she sucked in class, style, and eloquence. She didn't need to be told twice to do anything, most of all buying brand clothes.
Darcy, on the other hand... When he told her to dress well, she mocked him intolerably snob and kept on wearing her worn-out jeans. It took him a while to understand how to handle her. He couldn't tell her to do anything (unrelated to work), she would do the opposite. He couldn't also tell her to do the opposite, because she was too clever and got the trick. But he could buy her clothes as gifts and make Natasha teach her etiquette.
Natasha was a true gem. He didn't trust her, but he didn't trust anybody and he trusted her to be untrustworthy and break the rules if needed. Which was, when needed, a very valuable trait. He knew since it takes one to know one.
Nat was much more than Darcy's bodyguard. She was her friend, her mentor, her trustee. And his asset of handling a wife who was 21 years younger than him and had soft curves but a strong will. There was iron under all that bubbling liveliness, a strength matching his own. She would do great things in life.
He was sure one day she would leave him, too. But before that, they would have so much fun.