If the Joker knew Harry Potter fanfiction, it would be a very odd combination. He could potentially use the knowledge of magic to enhance his already dangerous and unpredictable nature. He might take inspiration from the Death Eaters and form his own group of 'Dark Followers' in Gotham. He could also use the idea of Horcruxes and try to create something similar to ensure his immortality in his battles against Batman. However, it's also possible that he would simply mock the concepts in the fanfiction, seeing them as too 'wholesome' compared to his own brand of chaos.
If Harry Potter becomes the Joker in fanfiction, he would probably start to question all the values he once held. The Joker is all about subverting order, so Harry might begin to undermine the Ministry of Magic and other institutions. He could use his knowledge of magic to play cruel pranks on people, not the innocent kind of pranks he used to do. His Gryffindor courage could be twisted into a reckless disregard for the safety of others.
In such a fanfiction, it would be a very strange and chaotic scenario. Harry Potter comes from a world of magic and the Joker from a world of dark, madcap crime. If the Joker were to stalk Harry, Harry would likely be extremely confused at first. He might try to use magic to fend off the Joker, but the Joker's unpredictability would make it difficult. It could lead to a lot of psychological warfare as Harry tries to figure out the Joker's motives and protect himself and his friends.
In such a fanfiction scenario, Harry Potter's personality would likely be extremely distorted. The Joker's chaotic and immoral nature would probably make Harry grow up with a warped sense of right and wrong. He might be more prone to using cruel and unethical means to achieve his goals, far from the noble and kind - hearted Harry we know from the original series.
If Harry was adopted by the Joker in fanfiction, it could lead to some really interesting character development. Harry might start to develop a warped sense of humor and a disregard for the law like the Joker. On the other hand, he could also be a source of conflict within the Joker's world. Maybe Harry would try to resist the Joker's influence and secretly work towards bringing him down. It would be a battle of wills between the two, with Harry having to use his wits to survive in the Joker's crazy world.
Well, if Harry were raised by the Joker in fanfiction, his personality would likely be a complete contrast to what we know of him. The Joker's influence would make him unpredictable. He might not value life in the same way as in the original Harry stories. Instead, he could be more interested in creating chaos and playing cruel jokes on people, much like the Joker does. It would also change his relationships, as his friends in the original story would probably be very wary of him or even become his enemies due to his new, Joker - influenced behavior.
In a fanfiction scenario where Harry Potter knows the Joker, it would be a very interesting crossover. Harry, being used to a certain code of ethics in the wizarding world, might be shocked by the Joker's complete lack of moral boundaries. He could attempt to reform the Joker using magic, perhaps a spell to make him see the error of his ways. On the other hand, the Joker might see Harry as just another plaything in his game of chaos and try to break Harry's spirit.
Well, it could be a total chaos. Harry Potter's magical world is all about order in a way, with Hogwarts rules and the fight between good and evil in a more traditional sense. The Joker, on the other hand, is the agent of anarchy. Maybe he'd start causing mayhem in Hogwarts, like turning the moving staircases into death traps or making the portraits go berserk. And Harry would have to use his magic in really unorthodox ways to stop him.
It would be really strange. Harry Potter is from a magical world of wizards and witches, while the Joker is a chaotic and often dark character from the Batman universe. Maybe in this fanfiction, Harry could have the Joker's unpredictable nature and use his magic in really devious ways.
Well, it could happen in a world where magic and the chaos of Gotham collide. Maybe Harry uses his magic to get into the Joker's mind and find the good that's left in him. He could use a spell like Legilimency to understand the Joker's past traumas and then use healing magic to start the process of redemption.
Well, in a fanfiction world, anything can be possible. Maybe Harry Potter uses his magic skills like the Killing Curse. But it's really a strange crossover. The Joker is from a completely different universe with no magic, and Harry Potter is from a world full of magic. It would be a very unique and perhaps disturbing story to see how Harry Potter overcomes the Joker's chaos and madness with his magic and finally kills him.