Joker

Jack Napier, permanently known as The Joker, is an obsessive and nihilistic criminal, crime boss and supervillain who is the arch nemesis of Batman and the most dangerous criminal in Gotham City. He has committed various crimes against Gotham, and even some of Batman's rogues gallery, most notably Black Mask. Joker has killed Jason Todd and paralyzed Barbara Gordon, with his ultimate desire to make Batman so enraged that he will snap and kill him. However Batman refuses to give into Joker's wish and refuses to break his moral code.

It was revealed in the prequel series, Lego Batman Villain Origins, that Jack Napier agreed to help two criminals break into his old work at Ace Chemicals. However, he received a phone call from the GCPD to inform him that his wife had died from testing a baby bottle heater. Napier didn't have interest on going through on the plan, however the criminals told him that backing out was not an option. When they break in, Jack is mistaken for the leader of the crime group because he was wearing the helmet of the Red Hood. The two criminals were gunned down and Jack is confronted by Batman.

However, Napier trips on his cape and falls into a vat of chemicals below. The chemicals bleached his skin white, dyed his hair green and stained his lips red. It also drove him insane and he became The Joker.