Another possibility is that Sirius could be a bit protective. If he thought Harry was in a situation that might be compromising or if he was worried about Hermione's influence on Harry (not in a bad way, but just in terms of keeping him safe), he might step in more forcefully. He could say something like 'Harry, I think it's time for you to go do something else' and then give Hermione a bit of a stern look, even though he might not really mean to be that harsh. It all depends on how the author of the fanfiction has developed his character and the relationships within the story.
Well, depending on the context of the fanfiction, Sirius could also be very understanding. If he knew that Harry and Hermione were just having a private conversation about important matters like their fight against Voldemort or schoolwork, he might just give them a knowing smile and say something like 'Carry on, then. Just making sure everything's all right.' He's a rather perceptive character, after all. And he trusts Harry a great deal, so he would likely give them the benefit of the doubt before assuming anything untoward.
One possible reaction of Sirius could be shock. He might stand there frozen for a moment, unable to believe what he's seeing. Another reaction could be anger. If Sirius has certain expectations or feelings about James and Regulus' relationship, he could get angry at finding them together. Or he could be confused, trying to figure out how and why this is happening between his friend James and his brother Regulus.
Sirius may also be angry at first. Given his protective nature towards Harry, if he misinterprets the situation as Draco causing trouble for Harry, he would likely step in assertively. But if it's a situation where Harry and Draco have a more positive or friendly interaction in the fanfiction, Sirius would have to re - evaluate his initial reaction.
It could potentially strain the relationship. If Sirius misinterprets the situation, it might create some tension. Harry and Hermione would feel like they need to explain themselves, and Sirius might be a bit standoffish until he gets the real story. For instance, if he thought they were hiding something from him, it could make things a bit rocky between them for a while.
Well, perhaps in the fanfiction, Sirius would take Hermione on adventures around the wizarding world. Since he was part of the Marauders, he'd know all kinds of hidden places. They could visit ancient magical sites, and along the way, Sirius would share stories about her parents if he knew them. This would not only give Hermione a broader knowledge of the wizarding world but also a sense of connection to her past. Also, he might train her to be a better dueler, preparing her for any threats that come her way in the future.
One reason could be Harry's impulsiveness. Harry often jumps into dangerous situations without thinking, and Sirius, being his godfather and having a protective nature, would shout at him to make him realize the risks. For instance, if Harry decides to go after a Death Eater alone.
Well, Ron could get angry. He has a strong loyalty to Harry, and seeing Harry with Draco, who he has always seen as an enemy, might make him feel betrayed. He might immediately jump to conclusions and start shouting or demanding an explanation from Harry.
One reason could be that Hermione is under a lot of stress in the fanfic, and she takes it out on Sirius. Also, she might have some misinformation about Sirius that makes her think less of him. For example, if there are rumors going around about Sirius' past actions, Hermione might believe them without really getting to know him better. Another possibility is that Hermione is trying to assert her authority in a situation where she feels it's being challenged, and Sirius just happens to be in her way.
In fanfiction, Hermione and Sirius could have a mentor - mentee relationship. Sirius, with his knowledge of the wizarding world and his rebellious nature, could offer Hermione a different perspective on magic and the fight against the dark side. He might share stories of his time in the Order of the Phoenix that would inspire Hermione in her own battles against evil.
Well, in fanfiction, anything can be possible. Maybe it could be a result of some time - turning magic gone wrong. Or perhaps there was a secret relationship between Sirius and Hermione's mother that was never revealed in the original story. It's all about creative interpretations in fanfiction.
One popular story might be 'Forbidden Bonds' which explores a unique relationship between them. It delves deep into their characters and how they might interact in a different way than in the original series. The author creates a lot of tension and drama, making it an engaging read.