It would probably include Harry's initial confusion when he first encounters Eevee. He might be used to magical creatures in his own world, but Eevee is something entirely new. There could be a lot of descriptions of Eevee's appearance, like its fluffy fur and big, innocent eyes.
Well, it could happen if there was some sort of magical cross - over event. Maybe a powerful wizard accidentally casts a spell that transports Harry into the world of Pokémon, specifically the world of Eevee. And there, he gets fascinated by the cute and diverse forms of Eevee.
Well, one key element is to have a unique take on the existing Harry Potter world. Maybe your OC (original character) could be a new student at Hogwarts with a special ability that's not common in the books. Another important part is to keep the magic system consistent. You can't just make up new spells that completely go against what J.K. Rowling has established. Also, the relationships your OC forms with the canon characters, like Harry, Hermione, and Ron, are crucial. For example, they could be friends, rivals, or even have a romantic interest in one of them.
If Harry Potter becomes a demon in fanfiction, his relationships would be severely affected. His friends like Ron and Hermione would be very confused and possibly afraid of him. They might try to find a way to reverse the transformation. And Hogwarts itself would be in chaos. The teachers would be divided on how to deal with him, some might want to help him while others might see him as a threat to the safety of the school.
Well, it would be a total chaos. He might use his magic skills to cause destruction. For example, he could turn Hogwarts into a dark fortress and rule over it with an iron fist. His friends would be shocked and probably try to stop him, leading to some really intense battles.
In such a fanfiction, Harry might gain enhanced physical abilities like super strength and speed. He could also have new magical powers related to his vampiric nature. For example, he might be able to hypnotize his enemies. His relationships with his friends would be severely tested. Hermione might try to find a way to reverse his vampirism while Ron could be initially afraid of him.
In this kind of fanfiction, Harry could potentially use the Hulk's strength to fight Voldemort in a very different way. He might be able to smash through dark magic barriers with ease, and his rage - like the Hulk's - could be a new source of power against the Death Eaters.
Imagine Harry Potter getting into creepypasta fanfiction. He could begin to incorporate some of the darker themes into his own view of magic. He might explore abandoned parts of Hogwarts that he thinks could be like the settings in creepypasta stories. His adventures could take a very different turn as he looks for signs of the uncanny and the terrifying within the familiar walls of the school. His perception of good and evil could also be distorted, as the lines are often blurred in creepypasta. This could lead to some really interesting, if not a bit disturbing, character development for Harry.
Well, if Harry Potter becomes a lich in fanfiction, he would likely gain immortality. But he might also lose some of his humanity. His appearance would probably change, becoming more skeletal and undead - like. His magic could take on a darker, more necromantic tone.
In such a fanfiction, Harry Potter as a ghost might have a very different perspective on the wizarding world. He could be able to pass through walls and observe things secretly. Maybe he would try to influence the living wizards in some mysterious ways, like guiding his friends to discover hidden secrets or solve mysteries that were left unsolved when he was alive.
He might gain more independence in his actions. For example, he could make decisions about his own education and training without the interference of the Ministry or Dumbledore. He could choose to explore magic in his own way, perhaps even study some dark arts to understand his enemies better, but not necessarily use them for evil.