Joker Junior (DC Animated)

Joker Junior (J.J. for short) was Robin (Tim Drake) after he was tortured and experimented on by the Joker with "shocks and serums" for weeks in the abandoned Arkham Asylum. He was brainwashed into a laughing maniac and made up to resemble the Joker himself, "adopted" by the Joker and Harley Quinn as their "son." The Joker's brainwashing was so complete that "J.J." even assisted Harley Quinn in her efforts to kill Batgirl, giving his "mother" her bazooka when she requested it. However, it was not enough to make Robin kill Batman, and so when the Joker handed "J.J." a gun loaded with "Joker toxin" to kill Batman with, he instead shot the Joker.