I was giving weapons to Izuna to level her up, and accidentally clicked on my Poison Oubai to give her. It was favorited, so the messaged popped up saying I couldn't do it, and like a complete dumbass, I simply uncheck it from favorites, when I meant to unselect the weapon itself from the list.
So now my level 100 Poison Oubai, with its maxed skills and two maxed Ougi, is gone for good. Damn it all. I really, REALLY wish there was an undo action button or something.