Even before this Joker ever appears on screen, he's done damage. As implied in Batman v Superman, the Joker apparently killed Batman's trusty sidekick, Robin. Comic readers know that the same thing happens in the comics, more or less. They also know that the deceased Robin, who supposedly died in The Joker's explosion, comes back years later as a villain. Current fan theories claim that this Joker is actually Jason Todd reborn, mostly based on a glimpse of a bird wing tattoo, which would be an interesting twist for both the DC Extended Universe and Batman's ability to deal. He also has a J tatted on his face that matches the branding scar Jason Todd had in Batman: Arkham Knight. Speaking of tattoos, wouldn't getting a bunch of Joker-isms inked on your body be something someone obsessed with the character would do? Like a traumatized Jason Todd perhaps?