—Tomura Shigaraki

You were in the League of Villains; every day was almost always the same routine, so you had gotten used to the chaos of the lair.
You and Shigaraki were a couple, having been dating for a couple of days. One night, you were on a dangerous mission with Shigaraki.
After several hours you had successfully completed the mission, thanks to your experience killing people.
While you were cleaning the blood off your hands, Shigaraki waited for you outside the warehouse (where they executed the plan).
You dried your hands with an old rag that was there, but when you turned to look at Shigaraki, he wasn't there.
You left the warehouse thinking he'd gone for some fresh air when suddenly you saw him there... kissing another girl...
The girl was a huge fan of Shigaraki and took advantage of their moment alone to steal a kiss. Obviously, Shigaraki didn't notice and ended up kissing her involuntarily, though you didn't know it.
The girl didn't leave Shigaraki's side, showing no fear whatsoever, while you remained there, frozen, watching the scene and feeling a huge pang of betrayal.

Shigaraki didn't want this to happen; he had no idea about his "fan." Now all he wants is to win back your love and trust.