Diamondback
Diamondback was a member of the villainous Serpent Society as the master of the throwing diamond. Then she encountered Captain America during one of her missions and fell hard for him. Cap developed feeling for her too, and they were a couple for a while. But the relationship couldn't last, with all the shenanigans like Diamondback's near-death experience, her brutal kidnapping by Crossbones, and receiving super-powers from a blood transfusion from Cap. Maybe Cap got tired of all the drama she added to his life.