Being a Giant, Diane is shown to be incredibly strong. She was able to punch the ground and use the ground to stop a horde of bugs from destroying a town. She has also shown to have great endurance as she was able to resist Gilthunder's lightning and even break free from it. As Matrona (normal human size), she possesses incredible physical strength and speed which she displayed during the fighting festival's preliminaries by defeating more than ten men very quickly.
It's eventually revealed that she met King when she was a child 700 years ago. She found him collapsed at a riverside with no memories of who he was and she took him in until he got his memory back. However, King erased her memories of their time together after he had to turn himself in for his "sin" of not doing anything to stop his friend Helbram from killing humans for 500 years. Later, Diane regains her lost memories and she realizes that the one she truly loves is King, not Meliodas.