In this kind of fanfiction, there could be a lot of possibilities. Maybe Harry would be overjoyed at first, feeling like he finally has a true family member besides his friends. He might try to protect his sister from the dark forces that he has always faced. For example, he would be extra cautious about Voldemort or his followers finding out about her.
In many fanfictions where Naruto discovers he has a sister, it often changes the entire dynamic of his story. It could give him a new sense of family and belonging that he lacked growing up. Maybe this sister was hidden away for some reason, like to protect her from Naruto's jinchuriki status bringing danger to her. This new relationship can also influence how Naruto approaches his ninja goals and his relationships with his friends.
At first, Harry would probably be in shock. He's been through so much in his life, and this would be a huge new development. He might be a bit hesitant and scared, not sure if he's ready to be a father. But as time goes on, his Gryffindor courage would kick in and he'd start to take on the role with determination.
One of the most interesting aspects in this type of fanfiction is seeing Harry's initial shock. He has grown up in a non - magical world, so suddenly realizing he's a wizard would be overwhelming. He might first doubt it, thinking it's some kind of joke or mistake. But as the story unfolds, he starts to see the evidence around him, like strange things happening that can't be explained by Muggle means.
The story might involve the child being from a past relationship that Harry had forgotten due to some spell or memory charm. As he finds out, he has to face the mother of the child as well. This could lead to a complex web of relationships and emotions. He might struggle with whether he can be a good father, considering his own difficult upbringing without his parents.
The sister could also bring some new conflicts. For instance, if she has a different view on how the ninja world should be run compared to Naruto. This could lead to arguments and growth for both of them. Additionally, she might have her own set of friends or allies that Naruto has to get to know and work with, which would expand the cast of characters in the fanfiction and create more complex storylines.
In some fanfics, Harry could find out he has distant relatives from the Potter family who live in a far - off land. These relatives could be part of a different wizarding culture. When Harry meets them, he gets to learn about different types of magic and different ways of living in the wizarding world. He might also discover that his family has a much bigger role to play in the overall wizarding history than he thought.
In this fanfiction, Harry could be thrown into a whole new world of Greek mythology. He might have to go on quests, just like the traditional demigods. His friends at Hogwarts would be really confused when he starts showing these new demigod - like abilities. And he'd have to learn about his new heritage, which would be a huge shock considering he thought he was just a wizard.
His relationship with Hermione and Ron would be different. They might be a bit jealous at first of his new powers. But they would also be curious and try to help him understand his demigod side.
One possible plot could be that Harry discovers he has some long - lost connection to the Malfoy family. Maybe he finds out through some ancient magic or a hidden family secret. This could lead to him having to re - evaluate his entire view of the Malfoys, especially Draco. He might start to understand Draco's actions in a different light and even feel a sense of kinship with him.