Disclaimer: Marion was never designed to be based on the Joker, but she was designed to be an ENTP, and this is incredibly accurate to how I have pictured her, despite a few differences.
"Everyone has a dark side. And some have a maniacal, psychopathic, schizophrenically mass-murdering entirely insane side. Of course, I’m referring to none other than Heath Leger’s portrayal of the Joker in The Dark Knight. Yes, he’s crazy. And yes, he’s an ENTP.
"It’s important to note that anyone suffering from the plethora of psychological issues the Joker suffers from can and will do feats of insanity that range from weird to sadistic, yet none with the flare and genius that are so unique to the ENTP.
"The Joker has one goal: to be entertained. The strange thing about the ENTP, crazy or not, is that they are often restless, feeling bored with the simple humdrum of everyday life where others might be satisfied. But the Joker is a Perciever, so naturally structure, order, and the like are a far cry from what he desires. Minions? Nothing more than pawns to accomplish his goals. He wants chaos, anarchy—to prove that the rest of the world is just as monstrous as he is at their core. So what is a demented ENTP to do but start blowing things up, foil the plans of the mob, and outwit and slaughter the authorities at every turn? Very quickly, however, he realizes there is little joy in the ease of conquering such simpletons. He wants more. Someone to challenge him, to give him purpose. ENTPs don’t just want to win. There is no pleasure where there is no rivalry. They want the prizefighter—someone who will match their intellect, to throw fists with an equal, and above all for that person to be their tireless toy.
"Finally, when all hope for a mind-mate suitable for his needs seems lost, the Joker finds his purpose.
"Of course, it takes another NT to match his brilliance, but other than that, Batman is his opposite in every other way; the order to his chaos, the introspection to his indiscretion, the justice to his mayhem. The ultimate challenge. In the Joker’s mind, it’s easy to manipulate the average person into committing atrocities. But Batman? No. Batman cannot simply be tricked, he has to be broken.
"And so the Joker formulates a plan. He will destroy everything Batman believes in, the ideals to which he holds. His love, his hope—all of it must be burned."