Martha Wayne
In one of DC’s many alternate universes, Flashpoint, Bruce Wayne is the one who’s killed while his parents survive the infamous shooting. Thomas Wayne goes on to become the Batman of his Earth, while Martha transforms into the Flashpoint’s Joker.
Unlike the original Joker, her motivations are pretty clear: she’s lashing out at the world for losing her son and her husband for failing to save him. It gives a unique perspective and clarity to the character that we don’t normally see. In a way, we can actually sympathize with her for her loss.
What this storyline did prove is one thing: there’s no Batman without Joker and vice versa. They need each other to coexist and are doomed to do this same dance until one of them dies.