The Clown Prince of Crime is Batman's deadliest and most unpredictable foe.  Rumored to be a failed stand-up comedian turned petty criminal who fell into a vat of chemical waste that turned his skin white and his hair green while escaping from the scene of a crime, The Joker is certifiably insane.  There's a totally twisted love/hate relationship for Batman operating in The Joker. Batman knows The Joker is mentally unbalanced, perhaps beyond help, but that doesn't make the end result of his actions any less deadly.