Ne: The Joker understands irony implicitly, and speaks in it as though it’s his native language. He’s good at figuring people out, (partially using his Fe, but also an intuitive sense of what’s going on behind the lines). He likes to enjoy himself (in a perverted kind of way) and sees humor in pretty much everything. The Joker likes possibility and sees it everywhere. He’s creative in his approaches, always has a new surprising and horrible mess to thrust on people. He’s a planner, but gets distracted by new possibilities so easily that he never sticks to his original plans (thus more surprises).
Ti: He care a lot about efficiency, and though he loves a reaction from people, he doesn’t hesitate to tell them to hurry up, or take matters into his own hands when his attempts to manipulate them fail. He’s an out of the box thinker who doesn’t let anything from outside change his warped internal logic. At the same time, he certainly has a clear grasp of logic. He likes to talk nonsense and toy with other people’s time.
Fe: The joker is good at reading people’s motivations and using them to torture them. He understands universal motivations better than individual motivations, which allows him to easily manipulate large masses. He’s detached from his emotions and from any sense of morality to an extent that it’s obvious his Fe isn’t a very healthy function.
Si: The Joker gathers information and analyses it from every possibility and angle to figure out how to use it to get what he wants (though information is not his primary tool for controlling people). He doesn’t really talk about his past, and when he does, it’s deliberately inconsistent.