Marriage of convenience? Nah- Katsuki had heard of it, but would never get himself involved in that. He doesn't want to start a family... yet. The right person for him- is already taken...
But you? Yeah, you've heard of a marriage of convenience. "Quirk marriage", as your father had called it, walking you down the aisle to some random bloke with a powerful quirk.
You've been married for a year now. Your husband- let's call him Mako, has a powerful water and ice quirk. Your quirk? Probably nothing to do with his. But your parents? Screw them- because they want power. How to get power when your old? Powerful grandkids.
...Your parents suck.
After a few months of Mako never being home on time, you found out he was cheating on you. But you? Oh god, you were pissed as hell. Why? Who knows? This man, who you don't even love, is cheating on you, and you're pissed off? Wtf?
So, later at night... home alone again... you were taking your anger out. Rather than going on a hero patrol and beating up some villains, you were throwing things around, breaking dishes and etc.
Katsuki, who is your best friend, finished his hero patrol, and decided to pay you a visit- see how you're doing. But- when he heard the dishes breaking, he grew a little concerned. He picked up the spair key from under a flowerpot and let himself in, walking to the kitchen- still fully suited in his hero costume.
You were in a rage; tossing glasses, kicking chairs... just taking your anger out. Hero work was stressing you out, your parents were annoying as hell- and now the husband who you hate has given you MORE REASONS to hate him!
You tossed a plate at the door, but stopped as you didn't hear it shatter. You turned around and saw Katsuki standing there, holding the plate right infront of his face. Apparently- he caught it before it cracked on his skull. "Well... Breaking dishes is one way to take anger out- So you do you, Punk."
Katsuki said, suprisingly softly as he even said his old nickname for you.