If the sister has a romantic relationship with one of the characters, it can add a whole new dimension to the plot. Love triangles can form, causing jealousy and tension among the characters. This can also drive character development as they have to navigate their feelings while dealing with the greater threats in the wizarding world, like the ever - present danger of the Death Eaters.
It can greatly affect the plot. For instance, if Harry is female, the way Voldemort views her could be different. He might underestimate her in some ways or be more intrigued by her. This could change the sequence of their confrontations. Also, if Hermione is male, his friendship with Ron and Harry could have new tensions. Maybe he'd be more competitive with Harry, which could lead to different sub - plots within the story.
His quitting potions could open up new storylines. He might have more time to explore other areas of magic or get involved in different adventures. For example, he could start an independent study of a unique form of magic that was not possible when he was occupied with potions. This can add more variety to the fanfiction plot.
It can completely change the direction of the plot. For instance, if Harry absorbs the Horcrux early on, he may have to face different challenges than in the original story. He might become a target for both sides, as the good side could be wary of his new, Horcrux - influenced state, and the bad side might see him as a threat to Voldemort's power.
In some fanfictions, it could be through powerful magic spells that he discovers or is taught. Maybe he stumbles upon an ancient spell in the restricted section of the library that allows him to summon a family member. Another possibility is that he unknowingly activates some kind of magical artifact that has the power to bring a sister into his life.
In a Hogwarts setting, the sister's character could develop in many ways. For starters, she might start off as a shy and introverted student, overshadowed by Harry's fame. But as she makes friends and discovers her own talents in classes like Potions or Transfiguration, she could gain confidence. Maybe she gets sorted into a different house than Harry, which forces her to step out of his shadow and develop her own identity within her house. For example, if she's in Ravenclaw, she could become known for her intelligence and love of learning, and start to form study groups and friendships based on that.
It can greatly change the plot. If Harry has a different name, like Ethan, for example, Voldemort might not target him as easily. This could delay the start of the main conflict in the story. Or, it could cause the Order of Phoenix to be more cautious in approaching him as they may not be sure if he's the 'Chosen One' with this new identity.
Some fans might be very disappointed as Ginny and Harry had a much - loved relationship in the original books. It could lead to a lot of debate within the community, with some fans strongly against such a plotline and others being more open to exploring different and darker aspects of relationships.
Often, the sister is depicted as having a similar mischievous streak as Loki. For example, she might play pranks on the other Asgardians, but with a different motivation. Maybe she does it to get attention or to prove herself in a male - dominated society like Asgard.
It can make the story more intense. A pissed - off Harry Potter is likely to take more drastic actions. For instance, he might be more aggressive in his fight against the dark forces, which can drive the plot forward quickly.
Well, it can completely change the story arc. Since Harry is a central figure, if he's uncaring, the whole fight against Voldemort could be affected. He may not be as motivated to seek out Horcruxes or train for battles.