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.
It would be a wild mix. Maybe Harry would inherit some of the Joker's chaotic nature while still having his magical abilities. He could use magic in really unexpected and devious ways, like the Joker uses his tricks.
The meeting between them in fanfiction could be quite intense. Harry Potter, coming from a world where good and evil are clearly defined by magic and the nature of spells, would be faced with the Joker, whose evil is based on pure insanity and a love for chaos. Harry might try to reason with the Joker at first, using his own sense of justice. But the Joker would probably just laugh at him and start creating havoc. This could lead to a battle of wits and powers, with Harry having to figure out how to deal with an enemy who doesn't follow the rules he's used to.
Well, if Harry Potter was raised by the Joker, he would probably be trained in a very different set of skills. Instead of learning magic in a traditional Hogwarts way, he could be taught how to be a master of chaos and trickery. His values would be completely different too. He might not have the same sense of justice and good that he has in the original story. The Joker's influence could lead Harry to question the very concept of right and wrong. And his relationships would be strange. He may not form the same kind of friendships as he did in the wizarding world. He might be more isolated or form alliances with those on the darker side of the magical or Muggle world.
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 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.
The plot could start with Harry somehow ending up in Gotham. He notices the Joker's chaos and decides to intervene. Harry might use his invisibility cloak to get close to the Joker without being detected at first. Then he could start using his magic to slowly disrupt the Joker's evil plans.
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.
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.
One consequence could be that the Joker's followers would be out for revenge. They might target Harry Potter or his friends if they also exist in that fanfiction world. Another consequence could be that the balance of power in the Joker's world is completely disrupted. There could be a power vacuum that other villains or even some anti - heroes might try to fill.
Harry would likely be very suspicious of The Joker from the start. He's used to dealing with threats in his own world, and The Joker's erratic behavior would set off alarm bells. Hermione would probably try to study The Joker, looking for a logical explanation for his actions and a way to counter him. Ron, on the other hand, might be a bit more fearful at first but would still stand by his friends. They would initially approach The Joker with caution, trying to figure out if he has any magical abilities or if he's just a very strange Muggle who has found his way into their world.