It could be that Ginny has gotten involved in some sort of trouble in the fanfiction. Harry, being her friend, is concerned and turns to Sirius, who has been through a lot in the wizarding world. Sirius can offer different perspectives on how to deal with the situation, whether it's related to Ginny's studies, her friendships, or something more dangerous. Harry values Sirius' input and believes he can help Ginny out of whatever predicament she's in.
Maybe Harry noticed Ginny acting strangely in the fanfiction and he wants Sirius' advice. Sirius has a lot of life experience and wisdom, so Harry might think he can help figure out what's going on with Ginny.
Ginny might be extremely embarrassed. She would probably blush deeply and try to hide behind Harry at first.
One great fanfiction could be 'The Unforeseen Bond'. In this story, Harry, Ginny and Sirius are brought together in a unique way. Sirius, who has a special connection with Harry, also starts to mentor Ginny, leading to some really interesting adventures. It's a story full of friendship and magic.
They could talk about Harry's parents. Sirius was Harry's father's best friend, so he could share many memories of James and Lily, like how they met at Hogwarts, their adventures in the Marauders' era, and what they were like as young wizards.
Their talk could also enhance Harry's magical knowledge. Sirius could teach him spells or magical techniques that he hadn't learned at Hogwarts yet. This new knowledge would be very useful for Harry in his fight against the Dark Arts. Moreover, learning from Sirius would make Harry more independent in his magical growth as he gets advice from someone outside of the school environment.
It could be a plot device to create drama. For example, if there's a love triangle situation and Harry faints on Ginny, it can stir up all sorts of emotions among the characters involved. It might also be because of some sort of dark magic that has been affecting Harry secretly and Ginny is the trigger point for him to finally succumb to it. This can then lead to a whole mystery - solving plotline in the fanfiction.
Jealousy could be a reason. If Ginny sees Harry getting too close to another female character, she might lash out and hurt him with her words. For instance, if Hermione and Harry were spending a lot of time together on a project, Ginny might feel left out and say something mean.
A misunderstanding. Harry might say or do something that Ginny misinterprets. For instance, if he makes a comment about her Quidditch skills in a way she thinks is condescending. So, she punches him to show her displeasure. And then the story could unfold as they clear up the misunderstanding.
In many fanfictions, if Harry and Ginny were caught by Sirius, Sirius might be initially protective and concerned. He could question their actions, especially if they were in a situation that seemed dangerous or inappropriate for their age. For example, if they were out after curfew or in a restricted area. Sirius might give them a stern lecture about being responsible and the importance of following rules, while also understanding their adventurous spirits as he was quite the rebel himself when he was young.
They might discuss Ginny's new-found magical abilities in the fanfiction. Maybe she has discovered a unique spell or has an extraordinary talent that's starting to surface.
Harry might blush and stammer. He could say something like, 'Sirius, it's not like that.' and then quickly try to change the subject.