Tim Drake (Joker Jr.)

Tim Drake (also known as Robin) was alone patrolling the streets of Gotham City when he offered himself to help a woman in trouble. After saving her, he is knocked out by her, who turns out to be Harley Quinn herself. After that, Drake is taken to the Joker's lair. Joker and Harley tortured Drake both physically and mentally so much, that he transformed into a young version of Joker. He is later forced by the Joker to kill Batman, but Tim manages to break for a little moment the brainwash imposed on him and kills the Joker instead. What he didn't count on, niether did Batman, was that during the torture he was submitted into, Joker implanted a microchip with his mental patterns.