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.
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 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 some fanfictions, Naruto could find out in a very emotional way. For example, his daughter might be in danger, and when he discovers she is his, his feelings of love and the need to protect her are intensified. He might also struggle with the fact that he has missed out on part of her life so far. He'd then start to make up for lost time by spending as much time with her as possible, teaching her ninjutsu and sharing his own life experiences.
He becomes more nurturing. Naruto, who has always had a big heart, now channels that into taking care of his daughter. He listens to her more intently, tries to understand her feelings, and is always there to offer support and advice, just like a good father should. He might also be more patient than he used to be. For example, if his daughter is having trouble learning a new ninjutsu technique, instead of getting frustrated like he might have in the past, he'll patiently guide her through it step by step.
In this kind of fanfiction, Naruto might initially be shocked. He would probably have to re - evaluate his whole identity. He has always seen himself as a ninja, but finding out he's a tailed beast would make him question his place in the ninja world. He might struggle with the idea that he is not just a human with a special power, but is in fact a tailed beast. This could lead to some interesting character development as he tries to come to terms with this new reality.
In some fanfictions, Naruto is completely devastated. He might go into a state of shock at first, unable to believe the news. Then he could be filled with intense grief and anger, blaming himself for not being there to protect Jiraiya.
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.
In many fanfictions, Naruto is initially in a state of shock. He can't believe that Jiraiya, his mentor, is gone. He might go into a period of deep sadness and isolation, withdrawing from others as he tries to process the loss.